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REVIEWS |
1 IOC Medical Commission and Karolinska Institutet, Stockholm, Sweden
2 IOC Medical Commission, Lausanne, Switzerland
3 Department of Orthopedics, University of Minnesota, Minnesota, USA
4 University of Vermont College of Medicine, Vermont, USA
5 Faculty of Sports Sciences, University of Waseda, Tokyo, Japan
6 Sports Medicine Centre, Duke University, Durham, North Carolina, USA
7 Department of Orthopedic Surgery, University of Ioannina, Ioannina, Greece
8 Cincinnati Children’s Sports Medicine Biomechanics Centre, Human Performance Laboratory, University of Cincinnati College of Medicine, Cincinnati, Ohio, USA
9 Oslo Sports Trauma Research Center, Norwegian School of Sports Sciences, Oslo, Norway
10 Chivas USA and LA Galaxy, Pepperdine University, FIFA Medical Committee, FMARC Member, Malibu, California, USA
11 Harvard Medical School Division of Sports Medicine, Children’s Hospital, Boston, USA
12 Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Denmark
13 Department of Orthopedics, Lund University, Lund, Sweden
14 IOC Medical and Scientific Department, Lausanne, Switzerland
15 Department of Exercise and Sports Science, University of North Carolina at Greensboro, North Carolina, USA
16 Stockholm Sports Trauma Research Center, Karolinska Institutet, Stockholm, Sweden
17 Medsport, University of Michigan, Michigan, USA
18 Scientific Activities IOC and Orthopaedic Center, Ullevaal University Hospital and Oslo Sports Trauma Research Center, Oslo, Norway
Correspondence to:
Professor Emeritus Per Renström, Flötviksvägen 51, Hässelby, 16572 Sweden; per.renstrom{at}telia.com
Accepted 14 April 2008
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 ABSTRACT EPIDEMIOLOGY OF ANTERIOR...RISK FACTORS FOR ACL...MECHANISMS FOR NON-CONTACT ACL...EVALUATION OF ACL INJURYMANAGEMENT OF ACL INJURYPREVENTION OF ACL INJURYOVERALL SUMMARY AND FUTURE...REFERENCES |
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As ACL injury remains a significant problem, especially in young female athletes, procedures for improved prevention and management are needed. The mechanism of ACL injury is an important focus of discussion, as an ACL tear is more often a non-contact event with a deceleration or a change of direction manoeuvre than a contact or direct blow injury. A prophylactic neuromuscular and proprioceptive training programme may reduce the number of ACL injuries in female athletes.
The President of the International Olympic Committee (IOC) Jacques Rogge stated in 2001 that "the most important goal of the IOC Medical Commission is to protect the health of the athlete". The IOC Medical Commission therefore invited a group of physicians, physical therapists, biomechanists and scientists active in ACL research to review current evidence relating to risk factors, prevention programmes and the need for further research concerning non-contact ACL injury in young female athletes.
| EPIDEMIOLOGY OF ANTERIOR CRUCIATE LIGAMENT INJURIES |
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ABSTRACTEPIDEMIOLOGY OF ANTERIOR...RISK FACTORS FOR ACL...MECHANISMS FOR NON-CONTACT ACL...EVALUATION OF ACL INJURYMANAGEMENT OF ACL INJURYPREVENTION OF ACL INJURYOVERALL SUMMARY AND FUTURE...REFERENCES |
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From the Norwegian data, a total of 2793 primary ACL ligament reconstruction operations were registered by 57 hospitals during 18 months. This corresponds to an annual population incidence of primary ACL reconstruction surgeries of 34 per 100 000 citizens (85 per 100 000 citizens in the main at-risk age group of 16–39 years). The number of ACL operations differs between the sexes in the second decade of life, with females having the most ACL reconstructions in the 15–19-year age group (fig 1).1
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ACL injuries: select sports
Looking at the ACL injury rates produced in select sports, the National Collegiate Athletics Association (NCAA) Injury Surveillance System (ISS) provides us with a unique 16-year sample of 15 sports across a college age group (typically 18–23 years).3 The ISS is a collection of data from representative colleges and universities (15% on average) and is not a registry. Approximately 5000 ACL injuries were reported over 16 years, producing an average of 313 injuries per year in the sample. Assuming the sample represents 15% of the total population, this equates to an average of more than 2000 ACL injuries in these 15 activities per year. If the number of reported injuries is considered, American football produced the greatest number of ACL injuries. However, if ACL injuries are ranked as a percentage of ACL injuries on a team compared with all injuries on that team, female sports dominate the list (female football/soccer, female lacrosse, female gymnastics and female basketball; fig 2A). If the ACL injury rate per 1000 exposures is considered, female gymnastics rate first with men’s spring American football second, closely followed by female football/soccer, female basketball and men’s in-season American football (fig 2B).
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The NCAA data report exposures as one session of sport (either practice or play) and do not record exposure in terms of hours. In this data set, the largest difference in injury rate (injury/athletic exposure x1000) between the sexes was in football/soccer (females twice that of men) and basketball (females three times that of men). Despite significant changes in the pace of the game of football/soccer and basketball and the improvement in sport-specific skills of females in general, the rate of non-contact ACL injury has remained stable over the 16-year period of NCAA data collection.4
For the age group slightly younger than college level (14–18 years), the rate of non-contact ACL injuries in soccer was twice as high in females as in males. For basketball the rate of injury in the younger female age group is the highest—nearly four times that of males. The ratio of male to female injuries decreases slightly at the college level and approximates 1 at the professional level (table 1).
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The best longitudinal data in this sport are from a 34-year case-control study in a single ski area in northern Vermont. During this time there were 6 780 940 skier visits and these were associated with 18 692 injuries, of which 2539 (14%) were ACL tears. Since 1991 there has been a 40% decrease in the rate of ACL injuries, with the incidence at the present time being one ACL injury per 3101 skier visits. Women constituted 40% of the control population but suffered 63% of all ACL injuries. The risk ratio for women sustaining ACL injuries averaged 2.5 times that of men in the time period of this study.5
Team handball
Several studies have reported the incidence of ACL injuries in team handball. A retrospective study found that the incidence of ACL injury was highest in women playing at the top level (0.82 ACL injuries/1000 playing hours) compared with 0.31 injuries/1000 playing hours in men. The relatively high incidence of ACL injuries in female players—particularly elite players—has been confirmed by later prospective studies.6 7 The highest incidence of ACL injuries occurred in female elite handball in Norway with 2.29 ACL injuries/1000 match hours. The incidence of ACL injury is high in team handball compared with other team sports such as football/soccer, basketball and volleyball. However, a direct comparison between the studies is difficult because different methods were used to collect and analyse the data and the way in which the incidence of ACL injuries was reported was inconsistent. Some report the number of injuries per 1000 playing hours while others report the number of ACL injuries/1000 athlete exposures. Nevertheless, all studies have found that women have a higher incidence of ACL injuries than men.
The incidence rate of ACL injuries in all sports studied appears to be significantly greater during competition than during training and this finding is consistent among sports.6
| RISK FACTORS FOR ACL INJURY |
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ABSTRACTEPIDEMIOLOGY OF ANTERIOR...RISK FACTORS FOR ACL...MECHANISMS FOR NON-CONTACT ACL...EVALUATION OF ACL INJURYMANAGEMENT OF ACL INJURYPREVENTION OF ACL INJURYOVERALL SUMMARY AND FUTURE...REFERENCES |
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The risk factors for ACL injury have been considered as either internal or external to an individual. External risk factors include type of competition, footwear and surface, and environmental conditions. Internal risk factors include anatomical, hormonal and neuromuscular risk factors.
External risk factors
Competition in games versus practice
Very little is known about the effect of type of competition on the risk of an athlete suffering ACL injury. Myklebust et al8 reported that athletes are at a higher risk of suffering an ACL injury during a game than during practice. This finding introduces the hypothesis that the level of competition, the way in which an athlete competes, or some combination of the two increases an athlete’s risk of suffering an ACL injury.
Footwear and playing surface
Although increasing the coefficient of friction between the sports shoe and playing surface may improve traction and sports performance, it also has the potential to increase the risk of injury to the ACL. Lambson et al9 found that the risk of suffering an ACL injury is greater in football athletes who have boots with a higher number of cleats and an associated higher torsional resistance at the foot-turf interface. Olsen et al10 reported that the risk of suffering an ACL injury is greater in female team handball athletes who compete on artificial floors that have a higher torsional resistance at the foot-floor interface than in those who compete on wood floors. This relationship did not exist for male athletes.
Protective equipment
Functional bracing appears to protect the ACL-deficient knee of alpine skiers from repeated injury; however, the effect of these braces on an ACL graft is inconclusive and requires more study. Kocher et al11 studied professional skiers with ACL-deficient knees and found a greater risk of knee injury in those who did not wear a functional brace than in those who did use a brace (risk ratio 6.4). McDevitt et al12 performed a randomised controlled study of the use of functional braces in cadets attending the US military academies who underwent ACL reconstruction. At the 1-year follow-up the use of functional bracing did not affect the rate of ACL graft re-injury. It is important to point out that there were only three injuries among those in the unbraced group and two injuries in the braced group. This was an impressive study that required considerable work, and a larger sample size that produces more re-injuries may carry with it the capability to determine whether or not functional bracing can reduce the likelihood of ACL graft injury.
Meteorological conditions
For sports that are played on natural or artificial turf, the mechanical interface between the foot and playing surface is highly dependent on the meteorological conditions. However, very little is known about the effect of these variables on an athlete’s risk of suffering an ACL injury. Orchard et al13 reported that non-contact ACL injuries sustained during Australian football were more common during periods of low rainfall and high evaporation. This work introduces the hypothesis that meteorological conditions have a direct effect on the mechanical interface (or traction) between the shoe and playing surface, and this in turn has a direct effect on the likelihood of an athlete suffering an ACL injury.
Summary and future directions for external risk factors
Internal risk factors
Anatomical risk factors
Abnormal posture and lower extremity alignment (eg, hip, knee and ankle) may predispose an individual to ACL injury by contributing to increased ACL strain values; alignment of the entire lower extremity should therefore be considered when assessing risk factors for ACL injury. Unfortunately, very few studies have studied alignment of the entire lower extremity and determined how it is related to the risk of ACL injury. Most of what is known has come from investigations of specific anatomical measures.
Notch size and ACL geometry
The dimensions of the intercondylar notch have been the most discussed anatomical feature in the published literature in relation to acute ACL injuries. Geometric differences in the size and shape of the ACL have not been well characterised. In general, studies on ACL geometry and notch dimensions are difficult to interpret because of the lack of standardised methods to obtain the data. Despite the number of methods for measuring the notch, notch width measurement of bilateral knees with ACL injury is smaller than that of unilateral knees with ACL injury, and notch widths of bilateral and unilateral knees with injury to the ACL is smaller than notch widths of normal controls. This implies a strong association between notch width and ACL injury.15
In trying to understand the relationship between a small notch and the risk of ACL injury, the size of the ACL has been reviewed. Methods to determine both notch size and ACL dimensions include radiographic, MRI and photographic techniques. Despite these differences, recent reports have concluded that the ACL is geometrically smaller in women than in men when normalised by body mass index. The properties of the ACL material may differ between the sexes, and there might be an additional link to the association between notch width/ACL size and ACL injury.
Posterior tibial slope
A highly significant correlation has been reported between the posterior inferior tibial slope and anterior tibial translation.15 It is well known in the veterinary literature that tibial plateau levelling osteotomies are of value for the treatment of cranial cruciate ligament ruptures.16 Although two small studies did not find a relationship between non-contact ACL injuries and the caudal slope of the tibia,15 17 there is new evidence to suggest that more specific measurement of the lateral tibial slope angle by MRI might be of value. One recent case-control study18 suggested that subjects with ACL-deficient knees had a significantly greater slope of the lateral tibial plateau and a lower slope of the medial tibial plateau than a control group. This paper suggests that the tibial slope of the medial and lateral condyle should be compared separately.18 The causal relationship between the posterior slope of the tibial plateau and ACL injury, pivot shift grade, risk of re-injury and potential risk of arthritis remain areas for future research.
Summary and future directions for anatomical risk factors
Hormonal risk factors
Structure and mechanical properties of the ACL
Studies characterising the structure and mechanical properties of the ACL indicate that women have smaller ACLs that may have lower tensile linear stiffness, are characterised by less elongation at failure and lower energy absorption and load at failure than men.19 20 These differences are not explained entirely by dimensional characteristics (ie, difference in size between men and women). This is supported by similar sex differences in the structure and mechanical properties (greater elongation, greater strain and lower stiffness in women) of the medial gastrocnemius21 and patellar22 tendons, which are also not explained entirely by anatomical differences and tendon size.
At the more macroscopic level, women also have greater tibiofemoral joint laxity (ie, anterior knee laxity, genu recurvatum) and lower joint resistance to translation and rotation.23–27 These differences are not limited to the sagittal plane but are also reflected in frontal and transverse plane motion.28–30 Together, these findings indicate that a broad physiological mechanism (eg, hormone control, differences in articular geometry, collagen turnover) may explain these sex differences, which have been implicated in ACL injury.
Although the causes/mechanisms of these sex differences are not well understood, the identification of sex hormone receptors (eg, oestrogen, testosterone, relaxin) on the human ACL31–34 has prompted studies of the potential of sex hormones to affect the structure, metabolism and mechanical properties of the ACL. To date, no specific mechanism has been identified by which hormones influence the biology and physiology of the ACL, but there is enough evidence to suggest that hormones are significant factors in the normal biology and physiology of collagen, muscle and bone.
The conflicting findings to date are in large part due to our poor understanding of this complex system, and unfortunately the various models used to examine these relationships are often not directly comparable. To understand the full effects of hormones on soft tissue and ACL injury, researchers must:
It is clear that there are sex differences in muscle performance. It is well accepted that mechanical loading will affect the biology and physiology of tendons and ligaments. However, the extent to which absolute hormone concentrations or variations across the cycle augment or inhibit tissue remodelling in response to mechanical stress is yet to be understood.
ACL injury and the menstrual cycle
There appears to be a consensus emerging from the literature that the likelihood of incurring an ACL injury does not remain constant during the menstrual cycle, with a significantly greater risk during the preovulatory phase than during the postovulatory phase. Initial work by Wojtys et al35 used self-reported menstrual history data to characterise the menstrual status of a subject at the time of injury and demonstrated a significantly greater prevalence of non-contact ACL injuries among women athletes during the preovulatory phase of the menstrual cycle. In a subsequent study by the same group,37 urine levels of oestrogen, progesterone and luteinising hormone metabolites were used to characterise a subject’s menstrual status at the time of injury. This confirmed that significantly more ACL injuries occurred during days 9–14 of a 28-day cycle with fewer injuries than expected during the postovulatory phase (defined as day 15 to the end of the cycle). Arendt et al38 found that female athletes were at increased risk of suffering an ACL injury during the preovulatory phase of their menstrual cycle compared with the postovulatory phase. Slauterbeck et al39 also reported a disproportionally greater number of ACL injuries during the preovulatory phase of the menstrual cycle, with fewer injuries occurring as the cycle progressed. In a study of recreational alpine skiers, serum concentrations of progesterone and oestradiol were used to stage the phase of a skier’s menstrual cycle at the time of ACL injury.40 Skiers in the preovulatory phase of their menstrual cycle were significantly more likely to suffer an ACL tear than those in the postovulatory phase (odds ratio 3.22). A comparison between this investigation and the previous study by Wojtys et al revealed a striking similarity. In the study of recreational alpine skiers, 74% of the women with ACL injuries were in the preovulatory phase of their menstrual cycle and 26% were in the postovulatory phase. Likewise, Wojtys et al37 found that 72.5% of the women not using oral contraception experienced ACL injuries during the preovulatory phase of their menstrual cycle compared with 27.5% during the postovulatory phase. In contrast, Myklebust et al8 studied competitive European team handball players over 3 years and found an increased risk of ACL injury during the week before or just after the onset of menstruation.
The stabilising effect of oral contraceptives on the female hormone profile and our understanding of their ultimate impact (both acute and chronic) on soft tissue behaviour, injury or performance is yet to be defined. It is important to realise that the type (ie, progesterone only versus combined oestrogen and progesterone compounds) and dosage varies widely among oral contraceptives, and both endogenous and exogenous levels must be accounted for. There is no conclusive evidence that oral contraceptives have a protective effect specifically against ACL injury.
Summary of hormonal risk factors
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ABSTRACTEPIDEMIOLOGY OF ANTERIOR...RISK FACTORS FOR ACL...MECHANISMS FOR NON-CONTACT ACL...EVALUATION OF ACL INJURYMANAGEMENT OF ACL INJURYPREVENTION OF ACL INJURYOVERALL SUMMARY AND FUTURE...REFERENCES |
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Video analysis
Video analysis is essential as it is usually the only way to obtain kinematic information from the actual event. In rare cases, injuries have even occurred during biomechanical experiments, but for obvious ethical reasons we cannot base our research on this approach. Six studies have used video analysis to study non-contact ACL injury mechanisms in sports (table 2).43–48
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Unfortunately, the reported variables among the studies using the visual inspection approach are non-standardised, making it difficult to compare them. Another major limitation of these studies is that all except from one are based on visual inspection. Krosshaug et al49 validated the visual inspection method and found the accuracy and precision among six experienced ACL researchers to be poor. For example, it was found that the true knee flexion angle was generally twice as high as the estimate. When the analysts estimated 30° knee flexion, the true angle was 50–60°. Results from studies based on this method must therefore be interpreted with great caution.
To overcome the inherent difficulties of visual inspection of human motion from video, Krosshaug et al50 developed a novel and versatile technique using model-based image matching from one or more camera views. In this method, 3D models of the surroundings as well as a customised skeleton model are manually matched to the background video footage. This method proved to be much more accurate with a difference in root mean square from traditional marker-based motion analysis of <12° for hip and knee flexion/extension. In a new study, Krosshaug et al48 demonstrated the feasibility of this method to actual injury videos. Detailed time courses for joint kinematics and ground reaction force were obtained for a four-camera basketball video and a three-camera European team handball video. The valgus angle increased abruptly in both cases, from 4° to 15° within 30 ms and from 3° to 16° within 40 ms for the basketball and handball injury, respectively. However, in order to make general statements on typical injury kinematics, a systematic approach to collecting and analysing more injury videos is needed.
Biomechanical studies
Recent studies have consistently shown that the predominant forces that affect strain in the ACL are anterior-directed shear forces applied to the tibia (either from external sources such as an anterior-directed force applied to the back of the lower leg or through internal mechanisms such as contraction of the dominant quadriceps muscles with the knee near extension). Important contributions to ACL strain values come from forces applied in the coronal and transverse planes of the knee. Biomechanical data have come from cadaveric studies of knees, strain gauges placed in vivo at the time of surgery and from analytical modelling. Cadaveric studies show that anterior-directed shear forces create most strain in the ACL with some added increase from varus, valgus and internal rotation moments.51–53 External torque applied to the knee produces relatively low ACL strain values. Valgus torque alone creates ACL strain only after significant injury to the medial collateral ligament. Interestingly, complete injury to the medial collateral ligament was necessary before significant injury to the ACL due to valgus torques applied in isolation.54 These cadaveric studies emphasise the importance of anterior shear forces in ACL injury.
With the knee near full extension, contraction of the quadriceps muscle—and the resulting force developed in the patellar tendon—produces an anterior-directed shear force on the proximal aspect of the tibia that strains the ACL. The ACL strain values are proportional to the magnitude of force in the patellar tendon and the angle of flexion of the knee. The patella tendon forms an angle with the tibia and its geometric orientation is able to produce ACL strain when the knee is near extension. An impulsive load applied to the patellar tendon with the knee in slight flexion has the potential to create an injury to the ACL.55 Application of patellar tendon force alone could cause enough ACL strain to alter it grossly and to increase anterior knee laxity. Application of impulse to a cadaveric knee flexed at 25° increased strain in the ACL. When the impulse was applied 15° out of the knee flexion plane to create a valgus force, the amount of strain was increased.56
In vivo studies have produced similar conclusions. Strain gauges were placed on the ACL after arthroscopic surgery in which the ACL was normal. The gauges were left in place long enough for the subjects to recover and perform rehabilitation exercises with moderate contraction of the leg musculature.57 Strain in the ACL is affected by anterior-directed shear loads applied to the tibia (relative to the femur) and a little by varus/valgus torques. Rehabilitation exercises can also produce significant strain in the ACL. Exercises that include contracfotion of the quadriceps in isolation with the knee near full extension produce high ACL strain values, while co-contraction of the hamstring muscles in combination with contraction of the dominant quadriceps muscles reduces ACL strain values in comparison to contraction of the quadriceps muscle group in isolation. Strain gauges have been implanted in subjects undergoing a high speed stop jump. The ACL was strained before heel strike and reached a maximum value very close to the maximal ground reaction force at landing.58 Cadaveric and in vivo studies have not clearly elucidated the biomechanical factors seen in the motion analysis studies of ACL injury. These studies often show a prominent motion in the coronal plane that involves femoral adduction and internal rotation, knee flexion and valgus, tibial rotation, and foot and ankle valgus. Motion analysis studies show a complex connected combination of movements which appear as an apparent valgus in the coronal plane but probably have only a relatively small contribution from actual medial opening of the knee, which would injure the medial collateral ligament if all the apparent valgus was isolated to coronal plane knee motion.
Kinematic analysis
Kinematic analysis of athletic college students revealed several differences in motion during running, side-step cutting and cross-over cutting. Women landed with less knee flexion and maintained a straighter knee during the entire stance phase. In addition to flexion, women displayed more valgus through the entire stance phase. It should be pointed out that the flexion occurs at the knee itself, while the valgus seen in the coronal plane could not have come entirely from rotation of the tibia around a stationary femur in the coronal plane but includes hip internal and external rotation combined with knee flexion. The muscle firing was assessed by electromyography (EMG). The quadriceps EMG was higher in women, with activation levels nearly twice those of the maximum EMG levels in muscle under maximum loading conditions in a dynamometer. Hamstring activity was less in women and the EMG values were only about half the level of a maximum measured by a dynamometer. ACL injuries appear to occur with hard and awkward landings on the knee when it is positioned near extension. The female knee is in more extension and has higher quadriceps activation at initial contact with the ground. The straighter knee and the higher quadriceps activation can combine to produce more strain in the ACL.59
A kinetic and kinematic analysis was performed for men and women performing a forward jump with a stop on a force plate followed by a jump slightly behind, straight above, or slightly ahead of the initial landing location.60 Inverse dynamics allowed computation of anterior shear forces which would act to produce strain on the ACL. Women had greater anterior-directed shear forces on the tibia for all jump conditions. Maximum ACL strains were observed near the initial contact period.61 Fatigue studies using a similar kinetic analysis showed that fatigue increased anterior-directed shear forces in both men and women.62
A kinematic study of young male and female football/soccer players showed that boys and girls land from a stop jump with a similar amount of knee flexion and valgus at ages 11 and 12. However, women begin to land with straighter knees progressively up to age 16.63
A mechanism seen frequently in ACL injuries involves a hard and awkward landing with the knee near extension (eg, straight). The hard landing implies high forces that are not "dampened" by hip, knee and foot motions. The "awkward’ part of the landing may involve landing outside the anticipated landing manner with higher than usual abduction or valgus moments applied about the hip and knee joints.64
Summary and future directions for mechanisms of ACL injury
Lower extremity valgus (knee abduction) loading and anterior tibial translation are likely to be involved in the mechanism. Future research should combine several research approaches to validate the findings such as video analysis, clinical studies, laboratory motion analysis, cadaver simulation and mathematical simulation.
| EVALUATION OF ACL INJURY |
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ABSTRACTEPIDEMIOLOGY OF ANTERIOR...RISK FACTORS FOR ACL...MECHANISMS FOR NON-CONTACT ACL...EVALUATION OF ACL INJURYMANAGEMENT OF ACL INJURYPREVENTION OF ACL INJURYOVERALL SUMMARY AND FUTURE...REFERENCES |
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Natural history
There is a little knowledge about the natural history of an ACL injury. The true incidence is probably not known. With MRI as a diagnostic tool the incidence is 8 per 10 000, which has been verified by register studies.1 65 However, it can be assumed that many patients with ACL injuries are not diagnosed at the time of the injury. A proportion of these athletes will present with knee symptoms or after re-injury. Others will never be diagnosed and are either without symptoms or have adapted to the altered knee function. The proportion of these categories is unknown. In a US community-based cohort there was a prevalence of ACL injuries of 4.8% in individuals aged 50–90 years unselected for problems.65
The course of an ACL injury depends on many different factors such as trauma mechanism, associated injuries, anatomical factors and activity level after the injury. There is no evidence of a sex difference. Independent of treatment, some patients will cope with their desired activity level ("copers"), some will adapt to an activity level adjusted for the knee function ("adaptors") and others will be unable to cope with being ACL-deficient ("non-copers"). Earlier reports show that each of these groups constitutes about one-third of the population.66 This is supported by a recent study of primarily non-operatively treated ACLs.67
Summary and future directions for evaluation of ACL injury
Associated injuries
As evaluated by MRI, an isolated ACL is an infrequent phenomenon although associated ligament injuries may not be very common.65 In the Norwegian National Knee Ligament Registry (NLKR), a total of 2793 primary cruciate ligament reconstruction surgeries were registered by 57 hospitals.1 In 27 cases (1%) a lateral collateral ligament injury was reported, while a medial collateral ligament injury was reported in 129 cases (5%). Non-operative treatment of medial collateral ligament injury is effective if combined with reconstruction of the ACL.68
Knees with acute ACL injuries should be evaluated for meniscus tears as these are identified in approximately 50% of cases.69 Lateral meniscus tears are more frequent in the acute setting while medial meniscus tears are associated with chronic ACL deficiency.70 In the NLKR there were a total of 1287 (47%) associated meniscus tears, 90% of which were treated surgically.1 In a study of ACL injury in 151 patients (36% women), 38% had an associated medial meniscus tear.65 ACL injury is a major long-term risk factor for the development of osteoarthritis, and no currently available treatment has been shown to reduce this risk factor.71 It is not clear if it is the associated meniscus tear at baseline, the development of a secondary meniscus injury, the meniscectomy or all of these that constitute risk factors for the development of osteoarthritis of the knee.72
Articular cartilage lesions were reported in 712 knees (26%) in the NKLR, 59% of which were treated surgically.1 When grading the cartilage lesions according to the International Cartilage Repair Society classification,73 222 (31%) cases were classified as grade 1, 283 (40%) as grade 2, 151 (21%) as grade 3 and 49 (7%) as grade 4 (grading was missing in 7 cases).
MRI is useful in assessing associated injuries such as bone contusions, intra-articular fractures and associated ligament injuries. One recent study has mapped out concomitant fractures and meniscus injuries and traumatic bone marrow lesions in the acute ACL injured knee by MRI and quantitative MRI (qMRI) in 121 subjects (26% women). Most of the ACL injured knees had a cortical depression fracture which was associated with larger volumes of bone marrow lesions.74
Clinical examination
Physical examination
Examinations appear to detect differences in end point better than differences in displacement.75 Joint motion varies within a normal population but there is little left-to-right variation in normal subjects. The ACL functions as a primary stabiliser to limit anterior tibial displacement and a secondary stabiliser to restrain tibial motion. The accuracy of diagnostic tests for ACL injury has been questioned. A positive pivot shift test result is best for ruling in an ACL rupture, whereas a negative Lachman test result is best for ruling out an ACL rupture. It is also concluded that, using sensitivity and specificity values only, the Lachman test is better overall for ruling in and ruling out ACL ruptures.76
The first clinical examination after an acute knee trauma has a low diagnostic value. For isolated ACLs, physical examination continues to have high specificity. Instrumented testing devices and stress radiography are used for objective measures but their use seems to be declining. Further assessment with MRI improves the chances of a correct diagnosis of intra-articular pathology and is recommended in the early phase after rotational knee trauma.65 The accuracy of MRI in predicting ACL injuries is 95% or more.
Outcome scores and outcome after ACL injury
During the last decade, consensus has been reached within the field of sports medicine, orthopaedics and general medicine that the primary outcome measure in clinical trials should be patient-relevant. Thus patient-reported outcomes with different focus have been developed, usually categorised as generic, joint-specific or disease-specific. For patients with ACL injury, disease-specific, joint-specific and generic instruments are used. Books and literature reviews outlining and comparing the currently available instruments for patients with knee complaints have been published.77–80
Validity, reliability and responsiveness
The most important property of an outcome measure is the responsiveness. For a measure to show high responsiveness in clinical studies, good validity and reliability are necessary prerequisites. A safe choice is therefore to choose an outcome measure with proven large effect sizes.
Validity is not an absolute or dichotomous property, and validation is a continuously ongoing process. The correlation between knee pathology and the patient’s perception is generally weak,81 indicating that, for patient-relevant outcome measures, the more important aspect is that the measure is validated for patients of similar age, sex and physical activity level rather than the specific knee pathology. A further fact in support of this view is that knee pathologies such as ACL tear, meniscal tear or cartilage injury often coincide and symptoms from specific pathologies cannot be separated.82
Score aggregation and score categorisation
Structural outcomes such as joint laxity and radiographic changes do not correlate well with patient-relevant aspects such as pain and function. Aggregating these outcomes into one total score will jeopardise interpretation of the results, and outcomes on different levels need to be reported separately.
Tapper and Hoover83 introduced a system for evaluation of symptoms and function following meniscectomy in 1969 in which they categorised outcome into four categories: excellent, good, fair and poor. This approach is appealing and the raw scores of established knee scoring scales are frequently categorised into these same four categories using arbitrarily chosen cut-off values. However, categorisation can introduce bias and lack of precision. The Lysholm scoring scale78 is a commonly used knee injury scoring scale which aggregates function and symptoms into one single score from 0 to 100 (worst to best). A cut-off of 84 points is used to categorise a good/excellent outcome. But what does a Lysholm score of 84 really represent? A patient could have a slight limp, some problems with stairs and some pain and be categorised as good/excellent with a total score of 89 out of 100. Alternatively, a patient could experience frequent instability that precludes sports activity, have some pain and yet score 85 and be categorised as having a good/excellent outcome. Most probably the patients themselves would not categorise either of these scenarios as a good/excellent outcome. Categorising the raw scores of rating scales tends to inflate the result, and the interpretation of categorical data depends on the content of the particular rating scale and the relative weight given to each component aggregated into the total score.84 Individuals rated as excellent or good on one scale may therefore be rated as only fair on another scale. Avoidance of data generalisation remains the optimal method for studying the outcome of knee injury.84
Administration mode of questionnaires
Substantial bias may be introduced if the operating surgeon answers the questionnaire.85 This also holds true for "unbiased" observers and applies to orthopaedic conditions other than ACL injury as well.86 87 Patient-relevant questionnaires should thus be self-administered (ie, filled out by the patients themselves in a neutral setting). The administration mode needs to be accounted for when comparing the patient-relevant outcomes between studies and over time within the same group of patients. The awareness of interviewer bias has increased over the years and more recently developed outcome measures are intended for self-administration.
Summary and future directions for outcome scores and outcomes
Motion analysis
ACL injury increases pathological anterior tibial translation and internal tibial rotation. Motion analysis enables detection of subtle kinetic and kinematic changes that may influence the final clinical outcome. There has been a gradual development of functional adaptations that protect the knee joint from excessive anterior translation episodes. Such a mechanism is limited quadriceps function near extension (the quadriceps avoidance pattern) and increased and prolonged hamstring activity.
Another possible mechanism of chondral damage is increased internal tibial rotation which can lead to excessive loads in cartilage areas not commonly loaded and thus contribute to future osteoarthritis. A better understanding of kinetic and kinematic changes and their final effect on the menisci and cartilage may provide useful information in the evaluation of an individual’s natural course.88–90
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ABSTRACTEPIDEMIOLOGY OF ANTERIOR...RISK FACTORS FOR ACL...MECHANISMS FOR NON-CONTACT ACL...EVALUATION OF ACL INJURYMANAGEMENT OF ACL INJURYPREVENTION OF ACL INJURYOVERALL SUMMARY AND FUTURE...REFERENCES |
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The major technical principles for an ACL reconstruction have become less controversial in recent years with improved knowledge of the anatomy, biomechanics and the healing process of the ACL following reconstruction. The choice of grafts has received much attention in the literature. In general there is little evidence to suggest significant differences between hamstring and bone-tendon-bone (BTB) grafts regarding short- and mid-term outcome.92 Reports of more degenerative changes in the tibiofemoral joint after ACL reconstruction with BTB grafts need to be verified in randomised clinical trials.93
An increased failure rate in women compared with men has been reported with the hamstring graft.94 In another study with questionable power, greater anterior knee laxity over time was found in women compared with men after reconstruction with the hamstring graft.95 These data may support the use of a BTB graft as the first choice in women but, importantly, it must be pointed out that there is no clear correlation between laxity and functional outcome.96
The double bundle technique for ACL reconstruction may have advantages regarding the long-term results since it seems to have the potential for diminishing rotational laxity.97 This may reduce the risk of later osteoarthritis but no data are available to support this theory. There is currently no convincing evidence that this technique results in better functional results than the single bundle procedures.
It appears that the incidence of ACL injuries among prepubescent children is similar in boys and girls.98 At puberty the incidence in girls becomes higher than in boys and, according to the Scandinavian ACL registries, more ACL reconstructions are performed on girls at this age.99 The principles for treatment of ACL injuries in this age group are not gender-specific.100
The high risk of secondary injuries after the initial injury must be considered and, independent of the treatment choice, these patients must be followed carefully. A specific problem is the potential risk of growth disturbances when reconstruction of the ACL is performed before puberty.101
Rehabilitation after ACL reconstruction
Motion and immediate weight bearing postoperatively have been standard since the 1970s.102 Pain inhibition is a common problem postoperatively (first days) and can result in inability to activate the thigh muscles, primarily the quadriceps, irrespective of the graft used, and the hamstrings when the hamstring graft is used. Electrical muscle stimulation may therefore play an important role in the early phase of rehabilitation after ACL surgery in order to stimulate activation of the muscles and is more effective in women than in men for preventing muscle hypotrophy.103
Failure to regain full knee extension after ACL reconstruction is the most common complication.104 Range of motion exercises with particular attention to regaining complete knee extension are therefore important and should be started during the first postoperative days. Full knee extension is individualised, with some patients having normal hyperextension. Knee extension of the contralateral leg should therefore be examined and the same degree of knee extension should be obtained in the ACL reconstructed knee.
Both closed kinetic chain (CKC) exercises and open kinetic chain (OKC) exercises for the quadriceps muscle should be used in the rehabilitation programme.105 OKC training should, however, be introduced cautiously, starting with a range of motion between 90° and 40° with a 10° increase every week. CKC exercises are recommended from the beginning of rehabilitation. A recent study showed greater anterior knee laxity when OKC quadriceps exercises were introduced 4 weeks after hamstring ACL reconstruction than when OKC exercises were introduced after 12 weeks.106 It is therefore recommended that OKC quadriceps training should not begin within 8 weeks of surgery. The same difference was, however, not found in patients with patellar tendon ACL reconstructions. Strengthening of the hamstrings may be difficult when one or more of the muscle tendons has been removed for the graft. Quadriceps strengthening is as important in patients with hamstring grafts as it is in patients with patellar tendon grafts. More problems were experienced with gaining strength of the hamstring muscles after ACL reconstruction with hamstring grafts than gaining strength of the quadriceps muscle after patellar tendon ACL reconstruction.107
A number of authors have emphasised the importance of neuromuscular training to restore knee joint stability and regain a normal motion pattern.108
When using functional performance tests for evaluation, it has been found that a battery of at least three (or four) different tests should be used.109 Patients should be tested during non-fatigued muscle conditions as well as fatigued conditions.110
There is as yet no evidence that rehabilitation after ACL reconstruction should be different for men and women. However, in 4890 patients with ACL reconstructions, increased laxity after reconstruction with a hamstring graft occurred independent of sex (Werner, unpublished 2008). No significant difference was seen between the sexes in improvement of quadriceps and hamstring muscle torques, one-leg hop test for distance, clinical outcome as evaluated by the Knee Osteoarthritis Outcome Score (KOOS) and activity level as evaluated by the Tegner Activity Scale 6–8 months after reconstruction and rehabilitation. The findings of increased laxity after ACL reconstruction with a hamstring graft may imply that rehabilitation should be slowed down after reconstruction with this graft.
Return to sport
Shelbourne and Gray111 stated that: "It has become almost universally accepted that rehabilitation of the healing ACL graft with rapid return to sports that have the potential to generate high stresses in the graft and knee does not lead any deleterious effects". Early return to sports in recent years has been advocated because of better surgical techniques and understanding of ACL biomechanics and the healing process.
Most articles published in recent years advocate a return to unrestricted sports 6 months or later following ACL reconstruction. Evidence-based evaluation has not proved that an early (2–4 months) return to sport is safe and efficacious, but it is possible to return to sports early (2–4 months) after ACL reconstruction.112 113 From the perspective of the graft and rehabilitation, it is also possible to do so with acutely injured ACL-deficient knees without surgery.114 115 A few articles have been published that advocate return to sports between 4–6 months. These are case series which are unable to prove that such an early return is more effective or safer than a more conservative rehabilitation.116 117
High-quality evaluations have shown that early weight bearing, early mobilisation, rehabilitation without braces and early CKC exercises are safe.118 Return to sports following ACL reconstruction should be based on regaining function, range of motion, strength and patient’s desire (goal-based, not time-based).112 118 119 However, in the long term, changes in cartilage metabolism noted following ACL reconstruction must also be considered. This is important when discussing the timing of a return to demanding activities after an ACL injury/reconstruction. Cartilage metabolic MRI and synovial fluid studies indicate that ACL-injured joints do not regain their normal glycosaminoglycan (GAG) structure until 2 years after the injury.120–
