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Frontal and lateral characteristics of the osseous configuration in chronic ankle instability
  1. O Magerkurth1,
  2. A Frigg2,
  3. B Hintermann3,
  4. W Dick2,
  5. V Valderrabano2
  1. 1Department of Radiology, University of Basel, Basel, Switzerland
  2. 2Department of Orthopaedic Surgery, University of Basel, Basel, Switzerland
  3. 3Department of Orthopaedic Surgery, Kantonsspital Liestal, Liestal, Switzerland
  1. Correspondence to Dr Arno Frigg, University Hospital Basel, Spitalstr. 21, CH-4031 Basel, Switzerland; mail{at}arnofrigg.com

Footnotes

  • Competing interests None.

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Several intrinsic (eg, hindfoot alignment, ligament laxity, neuromuscular control) and extrinsic risk factors (eg, shoes, type and intensity of sports, soil conditions) are known for chronic ankle instability (CAI).1 2 However, despite well-performed surgery, some patients with CAI will develop recurrence of instability (mechanical instability) and/or persistent subjective instability (functional instability). This problem has prompted many researchers to seek more intrinsic factors correlated with ankle instability.3 In a previous study, the osseous configuration of the ankle joint has been identified as another intrinsic risk factor for mechanical instability. The authors showed that a chronic unstable ankle might be characterised by a smaller tibiotalar sector, larger talar radius and increased talar height.4 These measurements have been made on lateral radiographs only. As generally accepted, a joint cannot be judged based on radiographs in one plane only, but a second plane is necessary. Furthermore, the clinical tests also assess the joint stability in two planes according to the ligaments: the anterior drawer test examines the anterior tibiofibular ligament (ATFL; representing an anteroposterior subluxation in the sagittal plane) and the lateral talar tilt test examines the calcaneofibular ligament (representing an inversion in the frontal plane).5 This is why a second plane is needed to determine the osseous configuration of chronic unstable ankles.

The aim of this study was to describe characteristics of the osseous ankle configuration in the frontal plane as well as to determine further characteristics in the sagittal plane according to biomechanical considerations.6 7 The hypotheses of this study were that (a) a deeper curvature of the talus in the frontal plane facilitates inversion injuries; (b) the lateral malleolus would be longer in CAI, causing a less parallel position of the ATFL to an antero-inferior-orientated luxation force; (c) the talar centre of rotation is not exactly in the tibial axis as has been assumed4 6; and (d) the lateral tibial surface angle would be smaller in CAI. Furthermore, criteria were established to predict the occurrence of an unstable osseous ankle configuration.

Methods

Study design

This study is a comparative case-control study of patients with symptomatic CAI and healthy subjects with a stable ankle. The study has been approved by the medical sciences ethical review board of the authors' university, and the subjects gave informed consent to participate in the study. The study was carried out in accordance with the World Medical Association Declaration of Helsinki.

Patients

Using the same cohort as in a previous report,4 a group of 52 patients (18 men and 34 women, average age 39 (13.9) years; table 1) with CAI were compared with an age-matched and sex-matched control group of 52 healthy subjects (18 men and 34 women, average age 37 (16.5) years, table 1). Inclusion criteria for patients with CAI were that they had at least three recurrent ankle sprains, with symptomatic instability requiring surgical ligament reconstruction. Patients for the control group were recruited among patients who were admitted to our emergency department for reasons other than ankle sprains or CAI and had radiographs of their ankles taken. The purpose of these radiographs was to rule out the possibility of ankle fractures caused by external forces. Patients were excluded if they had trauma to the lower extremity, acute ankle fracture, had previously undergone foot surgery, had hindfoot deformity or ankle osteoarthritis, posterior tibial dysfunction, limited dorsiflexion (<10°), neurological disorders or had any other systemic disease such as rheumatoid arthritis or diabetes. In addition, healthy subjects were excluded if they had acute or previous ankle sprain and/or any history of ankle instability.4

Table 1

Patient characteristics4

Radiological measurements

A DICOM/PACS application was used to carry out standardised measurements of the ankle joint on digital x rays. All ankle radiographs were performed in the radiology department of the authors' university on a digital flat panel system with flat detector technology (Aristos FX, Siemens, Erlangen, Germany). Weight-bearing standard anteroposterior and lateral radiographs with a long plate were taken by focusing the x ray exactly on the centre of the ankle joint. Only strictly lateral views were accepted, which were characterised by superimposition of the lateral and medial malleoli, while the distal fibula was projected onto the posterior third of the distal tibia and exact superimposition of the medial and lateral surfaces of the talus. The strict frontal radiographs were done in medial rotation of the foot so that the bimalleolar axis stands frontal on the films. All x rays were analysed by one independent and blinded radiologist on a radiology workstation with a high-resolution monitor using the DICOM/PACS review application E-Film (Department of Medical Imaging at the University Health Network and Mount Sinai Hospital in Toronto, Canada). The calibration of the measurements was obtained by radiographs taken of a phantom. Because of the setup of the x ray system with fixed tube-detector distance, the imaging was performed under constant parameters both for the phantom and for the patients to rule out different magnification factors.

In the frontal plane, the following parameters were measured:

  • The depth of the frontal curvature of the talus (froCu): measured as the maximal depth perpendicular to a line through the medial and lateral talar surface (fig 1).

  • The length of the lateral (laMa) and medial (meMa) malleolus: measured as the length from a line through the medial and lateral talar surface perpendicular to the ground to the tip of the lateral and medial ankle (fig 1).

Figure 1

Measurements of the osseous ankle configuration on frontal radiographs. froCu, depth of the frontal curvature of the talus; laMa, lateral malleolar length; meMa, medial malleolar length.

In the sagittal plane, the following parameters were measured:

  • The position of the talar centre of rotation to the anatomical axis of the tibia (TibCOR): a position of the centre of rotation anterior to the tibial axis was defined as positive, a posterior position as a negative value (fig 2).

  • The distance between the tip of the lateral malleolus and the talar centre of rotation: measured as the length of the lateral malleolus subtracted by the talar radius (fig 2).

  • The tibial lateral surface angle (TLS): the angle between the tibial axis and a line connecting the anterior and posterior borders of the tibia (fig 2).

Figure 2

Measurements of the osseous ankle configuration on lateral radiographs. FibCOR, distance between the tip of the fibula and the talar centre of rotation; TibCOR, distance between the tibial axis and the talar centre of rotation; TLS, tibial lateral surface angle.

Statistical methods

Statistical differences among groups were determined using the unpaired Student t test. Significance was considered at p≤0.05. All statistical calculations were performed using the STATISTICA statistical package (V.6.1, Tulsa, Oklahoma, USA, 2003). To establish criteria for an unstable osseous ankle configuration, sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) were calculated for parameters that were significantly different in both groups. Cutoff criteria varied within 1 SD from both means. Because the tibiotalar sector already included the influence of the radius,6 the radius was not evaluated separately.

Results

Radiological measurements

The depth of the frontal curvature of the talus was deeper in patients with CAI (1.8 (0.5) mm) than in healthy subjects (1.0 (0.5) mm, p<0.05, table 2). The length of the lateral malleolus was not significantly different in patients with CAI (24.4 (2.9) mm) compared with healthy subjects (24.0 (3.1) mm). The length of the medial malleolus was also not significantly different in patients with CAI (12.4 (1.7) mm) compared with healthy subjects (12.3 (2.0) mm). The position of the talar centre of rotation was more anterior to the anatomical axis of the tibia in patients with CAI (2.5 (1.9) mm) than in healthy subjects (1.6 (2.2) mm, p<0.05). The distance between the tip of the lateral malleolus and the talar centre of rotation was significantly smaller in patients with CAI (3.5 (3.4) mm) than in healthy subjects (6.5 (3.3) mm, p<0.05). The tibial lateral surface angle was not significantly different in patients with CAI (82° (3)°) compared with healthy subjects (83° (3)°, table 2).

Table 2

Radiological results of patients with CAI and healthy subjects

Criteria for an unstable osseous ankle configuration

Statistical assessments of the tibiotalar sector, froCu and TibCOR are listed in table 3. Because these criteria found a new approach to risk factors for CAI, cutoff criteria should allow a high PPV and specificity. To diagnose an unstable ankle configuration, a tibiotalar sector ≤80° can be used with a high PPV (83%) and high specificity (89%). However, to increase the sensitivity of this parameter, a sector of ≤82° can also be used with good statistical assessments (PPV 75%, specificity 75%; table 3). A frontal curvature ≥2 mm was determined as criterion for an unstable osseous ankle configuration (PPV 86%, specificity 88%; table 3). A TibCOR ≥3 or ≥4 mm (PPV 64% or 71%) dependent on the needed sensitivity can be considered as another criterion for an unstable ankle configuration (table 3).

Table 3

Criteria for assessment of a chronic unstable osseous ankle joint configuration: sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) for different parameters and cutoff points

Discussion

Following a former report,4 this study shows further characteristics of the osseous ankle configuration in CAI. These findings support that the osseous ankle configuration can be considered as an intrinsic, mechanical risk factor for CAI. In addition to a smaller tibiotalar sector, a larger radius and height of the talus,4 the osseous configuration of a chronic unstable ankle further exhibits a deeper frontal curvature of the talus and a more anterior position of the talar centre of rotation to the tibial axis. According to statistical considerations and feasibility in the clinical setting without a DICOM/PACS station, the following parameters can be considered most appropriate to anticipating chronic unstable osseous ankle configuration: (1) a tibiotalar sector ≤82°, (2) a frontal curvature ≥2 mm and (3) anterior TibCOR ≥4 mm. The tibiotalar sector is the most significant parameter because of biomechanical calculations6 and because it is a relative value that is not dependent on radiological magnification.

A lateral ankle sprain can be generally described as an injury in two planes: in the sagittal plane, it is an anterior subluxation of the talus out of the tibial mortise, whereas in the frontal plane, it is an inversion injury.5 8 Although the ankle joint also exhibits a slight inversion/eversion of 5° in normal gait, the frontal curvature of the talus was not measured yet.7 9 10 The current results show that a deeper froCu can facilitate inversion injury. This can be explained by the fact that a deeper curvature increases the bending amplitude, whereas in cases of a “flatter” curvature, the ligaments are protected by an osteochondral restraint of the talus itself (analogous to turning a ball or cube on the ground).7

The ATFL originates at the distal anterior fibula with its centre 10 mm proximal to the tip of the fibula and inserts on the talus 18 mm proximal to the subtalar joint. It has a slightly anterior–superior direction. It is 15–25 mm long, 6–8 mm wide, 2 mm thick and is known to have many variations.8 11 The ATFL becomes tight and more vertically orientated in plantar flexion and looser in dorsiflexion. Our hypothesis that a longer lateral malleolus, resulting in a less parallel position to an antero-inferior-orientated luxation force, would be associated with CAI could not be confirmed. McDermott et al12 also could not find anatomical differences of the ATFL in healthy subjects or those with CAI on MRIs or on anatomical dissection. However, our values are in accordance with measurements of the lateral and medial malleolar lengths in healthy subjects as shown by Fessy and Mariani: the lateral malleolus was 24–29 mm, the medial malleolus was 13–13.4 mm.7 13 The distance from the tip of the fibula to the talar centre of rotation was significantly smaller by 3 mm in patients with CAI (table 2). However, from a biomechanical point of view, this does not influence the ligament tension in an antero-inferior subluxation or in plantar flexion. It only would create a smaller torsional moment around the talar centre of rotation in case of a joint luxation.

So far, our studies have just focused on the bones and neglected the ligaments.4 6 Measuring the TibCOR allows an indirect measurement of the ligament laxity of the ankle joint on radiographs. In case of increased ligament laxity, the talus can subluxate anteriorly; the tibia is “hanging backwards” in the ligaments.

The ligament tension is very important because the ankle joint has a flexion-rolling mechanism similar to the knee joint. The talus must slide forward during plantar flexion and slide backward during dorsiflexion, which is guided by the ligaments as well the shape of the tibial and talar curves.14 15 In CAI, this sliding is not physiological anymore because of the damaged ligaments, which produces abnormal kinematics, increased loading and consequently accelerated degeneration.14 16 An eccentric position of the talus under the tibia therefore leads to increased stress, accelerated rates of degeneration and consequently ankle osteoarthritis.17 The anterior position of the talar centre of rotation to the tibial axis explains why CAI is leading to ankle osteoarthritis over an average time of 34 years.16 Furthermore, Tochigi et al18 has found an anterior subluxated talus compared with the tibial axis in cases of degenerative joint disease of the ankle, which is in accordance to our findings.

Several studies have reported a dorsal position of the fibula in the ankle mortise in CAI. These measurements have referenced the fibula on the talus.3 12 19 20 Scranton et al19 has found that the Ankle Malleolar Index (AMI) was larger in patients with CAI (17°) than in healthy subjects (9°). However, these measurements were done on non-weight-bearing CT scans potentially altering the results and referencing the fibula on the talus. Because biomechanical studies have demonstrated that incompetence of the lateral ligaments allows the talus to internally rotate and anteriorly subluxate on the tibia,21,,25 LeBrun and Krause assessed the fibula position referencing on the medial malleolus only.3 In this study, the authors did not find any significant difference in the fibula position, and they concluded that a posterior position of the fibula may not be a true pathological entity but rather the result of measuring an internally rotated talus. Considering the results of the current study, we can add a further reason for the measurement of an increased AMI in CAI: the dorsal position of the fibula can be explained as a consequence of the anterior subluxated talus compared with the tibia and fibula in CAI.

Clinical relevance

Patients with an unstable osseous ankle configuration are more prone to reinjury. Therefore, they require greater diligence in treatment after a sprain or surgery than individuals with a normal osseous configuration. The following recommendations can be made for the treatment of these patients:

  • As patients or athletes with an unstable osseous ankle configuration have a mechanical predisposition for recurrent ankle sprains, regular proprioceptive training is recommended with each training session and as a lifelong daily routine. Increased neuromuscular control has been demonstrated to be the best protection against sprains and to compensate for mechanical instability.26 27

  • Athletes known to have osseous instability should wear a brace, protective ankle-stabilising footwear and apply taping techniques to the ankle during sports. This has been shown to decrease instability by providing mechanical protection as well as stimulating the proprioceptive nerves.28

  • Adequate time for therapy and rehabilitation is needed for the injured athlete, usually 4–6 months.29 Professional athletes frequently experience a conflict between an inadequate short rehabilitation and the demands of their career. Taken in conjunction with a high risk for osseous sprains in competitive sports, this frequently becomes the start for the development of CAI and its complications, and ultimately can terminate the career of an athlete. In situations like this, the team doctor must arbitrate between his medical expertise and the interests of the sponsors, the team and the media. Bearing in mind the above-mentioned criteria for developing an unstable ankle configuration, the team doctor can structure a more adequate therapy and rehabilitation regime for a particular patient or athlete.

  • According to literature, treatment of grade III ankle sprains may consist of functional treatment (range of motion etc), immobilisation or primary ligament repair. Some authors recommend consideration of an early repair in professional athletes.30 A coexistent unstable ankle configuration may be a criteria to perform an early ligament repair in such cases.

  • Ventral ankle osteophytes are a sign of chronic instability as the joint tries to stabilise itself. In cases of anterior impingement syndrome, only anterior tibial osteophytes should be carefully removed as this procedure may destabilise the joint. Osteophytes should be primarily removed on the talar side if possible. Osteotomies or bone block procedures to increase the tibial coverage, similar to the treatment of shoulder instability, are simply theoretical.

Limitations of the study

The study has certain limitations:

  1. The ankle joint is a complex three-dimensional joint with a three-dimensional motion, and again, the question arises whether the ankle can be explained using radiographs in single plains.14 31,,33 This simplification of the ankle joint as a hinge joint has been widely accepted and a lot can be learned from this simple model as it applies to the clinical situation and the generally used plain radiographs.34 The proposed measured values on the radiographs are easily accessible in the clinical setting and during surgery compared with complex three-dimensional CT analysis. Furthermore, complex CT analysis does not show significant results comparing CAI and healthy subjects.35

  2. The measured differences of 0.9 mm in the TibCOR and 0.8 mm in the froCu are rather small and almost reach the lower detection rate of our digital devices. However, although the differences are small, they apply to the clinical observations and seem to be valuable in evaluating the anatomy of the chronic unstable ankle.

  3. There were more women in our study than men, which is in accordance to the susceptibility of women to sprains.36,,38 Other influencing factors (eg, activity level,39 body weight40) were not addressed, but because of randomly chosen patients in both groups as well as an age and sex match, their influence can be considered equal in both groups.

What is already known on this topic

Besides several known intrinsic and extrinsic risk factors for chronic ankle instability (CAI), the osseous ankle configuration has been recently identified as another risk factor. This unstable ankle configuration has been characterised in the lateral plane by a smaller tibiotalar sector ≤82°, a larger talar radius and larger talar height.

What this study adds

This study adds characteristics of the ankle configuration in CAI in the frontal plane (frontal curvature ≥2 mm). The study also shows an indirect measurement of ligament laxity in CAI represented by an anterior subluxation of the talus compared with the tibial axis ≥4 mm.

Conclusion

The osseous joint configuration is an intrinsic risk factor for CAI. In addition to a smaller tibiotalar sector, a larger radius and height of the talus, the chronic unstable ankle is characterised by an increased depth of the frontal curvature of the talus and an anterior position of the centre of rotation of the talus to the anatomical axis of the tibia.

Acknowledgments

This study was supported by the Swiss National Research Foundation, the Swiss Orthopaedic Society, the Lichtenstein-Foundation and the Academic Society of the University of Basel (FAG), Switzerland.

References

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Footnotes

  • Competing interests None.

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