Hip flexor muscle size, strength and recruitment pattern in patients with acetabular labral tears compared to healthy controls

Abstract

Acetabular labral tears are a source of hip pain and are considered to be a precursor to hip osteoarthritis. Hip flexor muscles contribute to hip joint stability and function but it is unknown if their size and function is altered in the presence of labral pathology. This study aimed to investigate hip flexor muscle size, strength and recruitment pattern in patients with hip labral pathology compared to control subjects. 12 subjects diagnosed with an unilateral acetabular labral tear were compared to 12 control subjects matched for age and gender. All subjects underwent magnetic resonance imaging (MRI) of their lumbo-pelvic region. Average muscle cross-sectional area (CSA) of the iliacus, psoas, iliopsoas, sartorius, tensor fascia latae and rectus femoris muscles were measured. Hip flexion strength was measured by an externally fixed dynamometer. Individual muscle recruitment pattern during a resisted hip flexion exercise task was measured by muscle functional MRI.

Hip flexor muscle strength was found to be decreased in patients with labral pathology compared to control subjects (p < 0.01). No difference between groups or sides was found for hip flexor muscle size (all p > 0.17) and recruitment pattern (all p > 0.53).

Decreased hip flexor muscle strength may affect physical function in patients with hip labral pathology by contributing to altered gait patterns and functional tasks. Clinical rehabilitation of these patients may need to include strengthening exercises for the hip flexor muscles.

Introduction

In recent times, there has been an increased awareness of hip joint pathology such as acetabular labral tears as a source of hip or groin pain in patients. Tears of the cartilaginous acetabular labrum are thought to lead to early degeneration of joint cartilage due to increased instability and repetitive shearing of the joint surfaces (McCarthy et al., 2001, Lewis and Sahrmann, 2006, Crawford et al., 2007). Apart from the proposed long-term consequences, acetabular labral tears have a significant impact on a patient's physical activity and function (Burnett et al., 2006). Currently, limited evidence exists for the most effective physiotherapy management for these patients (Orbell and Smith, 2011, Yazbek et al., 2011). For effective therapeutic exercise programs to be designed, more information is required about how this type of joint pathology may affect hip muscle structure and function.

The anterior hip flexor muscles contribute to hip joint function in gait (Andersson et al., 1997) and play an important role in stability and movement of the hip and pelvis (Andersson et al., 1995). Of the hip flexor muscles, the iliopsoas muscle is functionally important as it is thought to contribute to stability of the femoral head in the acetabulum through its muscle belly as it crosses the anterior hip joint (Philippon, 2001, Lewis et al., 2009). Along with the rectus femoris muscle, it is also thought to reinforce the anterior hip capsule (Shu and Safran, 2011). If hip joint stability is already compromised by the presence of a labral tear (Crawford et al., 2007), greater reliance may be placed on the anterior soft tissue structures to support the joint.

Changes in muscle structure and function can occur in response to joint injury and pathology. Previous research has found changes in hip muscle size and function with hip osteoarthritis (Arokoski et al., 2002, Rasch et al., 2007, Grimaldi et al., 2009a, Grimaldi et al., 2009b). For the hip flexor muscles, decreases in rectus femoris muscle size and hip flexor muscle strength were found in patients with advanced hip osteoarthritis (Arokoski et al., 2002, Rasch et al., 2007). In patients with femoroacetabular impingement (FAI), hip flexor muscle strength and tensor fascia latae (TFL) muscle activity were decreased (Casartelli et al., 2011). However, for patients with acetabular labral tears, it is unknown if changes in hip muscle size or function are associated with this type of joint pathology. No research to date has investigated individual hip flexor muscle size or recruitment pattern and hip flexor muscle strength in this patient population. Considering the role that these muscles play in hip joint function, investigation of the impact of this joint pathology on hip flexor muscle size and function is needed.

Muscle dysfunction within the hip flexor group could affect hip joint loading forces as previous research has shown that altered force contributions within the hip flexor muscle group resulted in increased hip joint forces (Lewis et al., 2007, Lewis et al., 2009). Investigating function of the individual anterior hip flexor muscles in previous research has required the use of invasive procedures such as fine wire electromyography (EMG) to measure activity of these muscles during specific tasks (Basmajian and Greenlaw, 1968, Andersson et al., 1995, Andersson et al., 1997). Muscle functional magnetic resonance imaging (mfMRI), which is non-invasive, is able to discriminate between activated and non-activated muscle after performance of an exercise (Fleckenstein et al., 1988, Meyer and Prior, 2000). The Magnetic Resonance (MR) signal arises from the precession of protons in a fixed magnetic field (Meyer and Prior, 2000). As the majority of protons in the human body are in the form of body water, regional changes in blood flow and extracellular fluid will influence signal intensity. By this means, the performance of an exercise will increase regional signal intensity in the activated muscle. This will be seen in MR images indicating that the muscle has been recruited during the exercise (Jenner et al., 1994, Meyer and Prior, 2000). Although it is not entirely understood, the effect is thought to arise from osmotically driven fluid shifts occurring as a result of cellular metabolism (Damon et al., 2007). mfMRI has been used by previous studies to depict the pattern of muscle recruitment during specific exercise tasks (Kumagai et al., 1997, Enocson et al., 2005, Baczkowski et al., 2006). Signal intensity changes on mfMRI were found to correlate with EMG measurements of muscle activity (Adams et al., 1992, Kumagai et al., 1997). mfMRI is considered to be a marker of muscle metabolic activity (Damon et al., 2007) and could be used to investigate the recruitment pattern of the anterior hip flexor muscles during an exercise task that is commonly used in rehabilitation.

Therefore, the aim of this study was to investigate individual hip flexor muscle size, hip flexor muscle strength and muscle recruitment pattern during an exercise task in patients with acetabular labral pathology compared to healthy subjects.