Internal snapping hip syndrome occurs due to sliding of the iliopsoas tendon over the iliopectineal ridge, femoral head, or iliopsoas bursa.1–4 The incidence of internal snapping caused by the iliopsoas tendon has been estimated to occur in 10% of the general population; however, the majority of these individuals are asymptomatic.5 Following release of the iliopsoas tendon, studies have noted an improvement in patient-reported outcomes.6–11 However, after release, weakness of hip flexion is a concern. Brandenburg et al12 reported a 19% reduction in seated hip flexion strength but no changes in supine hip flexion strength. Other studies have noted initial postoperative weakness that returned to normal by 8 weeks.10
The iliopsoas also plays a potential role as a dynamic stabilizer to the hip due to its anatomic location just anterior to the femoral head at the level of the hip joint, especially as the hip is brought into extension.13 Fabricant et al14 reported patients with excessive femoral anteversion (>25°) had lower Hip Outcome Score for Sport (HOS Sport) following arthroscopic release of the iliopsoas tendon than patients with normal femoral version. The authors postulated that this was secondary to the loss of the stabilizing effect of the iliopsoas muscle in patients predisposed to instability secondary to their increased femoral anteversion.
Previous studies have noted a strength loss of approximately 25% following arthroscopic release of the iliopsoas compared with the contralateral side, as well as the presence of muscle atrophy post-operatively.12,15,16 This study is unique because previous studies have not compared the area of the muscle-tendon unit before and after surgery on the same side; instead, the operative side was compared with the contralateral side. In addition, the postoperative character of the tenotomized tendon has not been analyzed for its morphologic changes after release. Therefore, the purpose of this study was to assess the change in muscle area proximally and distally, the amount of fat atrophy using the Goutallier-Fuchs grading system, and the character of the tenotomized tendon using advanced imaging after arthroscopic iliopsoas central compartment release.
Materials and Methods
After receiving institutional review board approval, a retrospective chart review was performed of all patients who underwent arthroscopic iliopsoas release between 2011 and 2016. Patients were included in the study if they had both preoperative and postoperative magnetic resonance imaging (MRI) of the ipsilateral hip, underwent hip arthroscopy that included arthroscopic iliopsoas release performed for symptomatic hip snapping, and had no previous surgical history on the ipsilateral hip prior to iliopsoas release. Postoperative imaging for continued hip symptoms thought to be out of the range of normal postoperative recovery was performed at the provider's discretion.
All images were reviewed by a fellowship-trained musculoskeletal radiologist (M.S.C.) with 19 years of experience. Cross-sectional area measurements were obtained using a computer-based clinical imaging program (Q-Reads; Mayo Clinic, Rochester, Minnesota) with a hand-traced circumference of the iliopsoas muscle on computed tomography (CT) or MRI scans. All measurements were assessed using an easily reproducible landmark in the axial plane using a sagittal reference image. The point used was the most distal level of the rectus femoris direct head attachment on the anterior inferior iliac spine (AIIS), which served as a reference landmark for both the pre- and postoperative studies (Figure 1). All measurements included the combined iliopsoas muscle area at this level (both the iliacus and psoas), as it was not possible to reliably separate these 2 structures.
Axial T1-weighted magnetic resonance images of the right hip showing the measurement of the iliopsoas at the level of the most distal insertion of the rectus femoris onto the anterior inferior iliac spine preoperatively (1391 mm2) (A) and postoperatively (923 mm2) (B) for a percentage change of 33.6%.
To evaluate intraobserver accuracy, a second measurement was obtained more than 10 days after the first measurement in random order on 15 hips. Studies have shown CT and MRI have strong agreement, reproducibility, and intrarater reliability in the context of skeletal muscle measurements.17–21 In cases with follow-up postoperative CT or MRI studies including the lumbar spine and pelvis, the psoas muscle cross-sectional areas also were measured comparing psoas areas between sides at the level of the distal most aspect of the L5 vertebral pedicle (Figure 2).
Axial T1-weighted magnetic resonance image of the lumbar spine showing the cross-sectional area of the psoas muscle measured bilaterally at the level of the distal most cut of the L5 vertebral pedicle. The left (L) side was the operative side, and the right (R) side was the nonoperative side.
The degree of muscular atrophy and fatty infiltration at this more superior level was graded using the method of Goutallier et al22 (Figure 3). Regarding the Goutallier-Fuchs classification system, grade 0 is minimal to no fat atrophy, grade 1 is moderate fat atrophy (more muscle than fat or the amount of muscle equals the amount of fat), and grade 2 is severe fat atrophy (more fat than muscle). Atrophy was not assessed distally because the amount of muscle belly at this level is minimal. The abbreviated Goutallier classification system has been validated and found to have better interobserver and intraobserver reliability than the 5-grade system.23 A subjective qualitative assessment of the morphology of the entire length of the postoperative psoas tendon was observed and recorded. This included identification of proximally and distally retracted tendon stumps, and the presence of definable fluid gaps or formation of any intervening scar tissue.
Axial T1 magnetic resonance images showing the different Goutallier-Fuchs (GF) grades for assessment of muscle atrophy. Grade 0 is minimal to no fat atrophy; the left side was the operative side (A). Grade 1 is moderate fat atrophy (more muscle than fat, or amount of muscle equals fat); the left side was the operative side (B). Grade 2 is severe fat atrophy (more fat than muscle); the left side was the operative side (C). White arrows point toward the left psoas muscle demonstrating the various levels of fat atrophy explained above.
The surgical technique consisted of hip arthroscopy performed with patients in the modified supine position using 2 to 3 portals.24 Central compartment arthroscopy was performed, after which the iliopsoas was released. The overlying capsule at the 3-o'clock position was debrided with a motorized shaver; the iliopsoas tendon then was identified and transected with a beaver blade.25 Accessory heads of the iliopsoas were searched for in each case to ensure complete tendinous release. All surrounding muscle fibers of the iliopsoas tendon were preserved with great care to minimize injury to non-snapping muscular tissue of the hip flexor.
Clinical relevance was assessed by prospectively collected patient-reported outcomes that were reviewed retrospectively for this study. The modified Harris Hip Score (mHHS) and the Hip Outcome Scores (HOS) subscales for activities of daily living (HOS ADL) and sport (HOS sport) obtained at least 1 year after surgery were verified and then averaged.
Statistical analyses were performed using simple correlations and paired t tests where appropriate. A multivariable linear regression analysis also was performed to determine whether there were any other factors contributing to the amount of atrophy seen on postoperative imaging, using sex, time between surgery and second postoperative MRI, and age at surgery as predictors. P<.05 was considered statistically significant. Statistical analyses were performed using SAS version 9 software (SAS Institute Inc, Cary, North Carolina).
A total of 46 hips (44 patients) met the inclusion criteria for the study; 35 (80%) were female and 9 (20%) were male, with 2 patients undergoing bilateral release. Mean time between the initial MRI and surgical intervention was 158.56 days (SD, 130.4 days; range, 21–601 days). Mean time between surgery and repeat MRI was 581.7 days (SD, 460.82 days; range, 71–2076 days). Mean time between the first and final MRI was 740.30 days (SD, 492.04 days; range, 71–2161 days)
Mean preoperative cross-sectional area of the iliopsoas at the level of the rectus femoris insertion at the AIIS measured 1184.9 mm2 (SD, 312.2 mm2). Postoperatively, mean cross-sectional area at the AIIS measured 993.5 mm2 (SD, 297.4 mm2), leading to a mean difference of 191.4 mm2; this difference was statistically significant (P<.0001). Mean percent change was 16% (SD, 11.1%) (P<.05). A repeat measurement of the cross-sectional area at the level of the AIIS was obtained more than 10 days after the initial measurement in a random order on 15 patients to check intraobserver reliability. Repeat interpretation had good intraobserver reliability with a mean correlation coefficient of 0.83.
Twenty-six patients had postoperative imaging that included the L5 vertebral body (16 MRI, 10 CT), which allowed evaluation of the psoas muscle belly at a more proximal level, comparison with the contralateral hip, and grading with the Goutallier-Fuchs classification system. One patient underwent bilateral release and was excluded from the contralateral comparison measurements. Mean cross-sectional area at the most inferior aspect of the L5 pedicle was 52% of the area of the contralateral side: 628.9 mm2 (SD, 277.5 mm2) on the surgical side vs 1216.2 mm2 (SD, 494.3 mm2) on the contralateral side. Mean difference was 587.3 mm2 (SD, 315.2 mm2); this difference was statistically significant (P<.0001).
Fat atrophy was evaluated on the operative side at the L5 pedicle level using the Goutallier-Fuchs classification system. In patients with advanced imaging of the entire pelvis postoperatively, mean grade of the muscle at the L5 pedicle was 1.3 (SD, 0.7; range, 0–2). Three hips (11.1%) were grade 0, 13 hips (48.1%) were grade 1, and 11 hips (40.7%) were grade 2.
Patient-reported outcomes were collected for 56% of the patients. Mean mHHS, HOS ADL, and HOS Sport scores for patients at least 1 year after surgery were 51.02 (SD, 19.6), 55.70 (SD, 19.8), and 41.73 (SD, 28.5), respectively.
Using Pearson coefficients, it was determined length of time between surgery and postoperative imaging was not correlated to either the amount of atrophy or the Goutallier-Fuchs grade. In addition, multivariate regression was performed to determine whether any other factors contributed to the amount of atrophy present postoperatively. The factors considered were age, length of time between surgery and advanced imaging, and sex. The amount of atrophy observed postoperatively was not dependent on any of these variables.
Qualitative assessment of the morphology of the postoperative psoas tendon was similar for all of the patients. There was moderate tendon thickening above and below the level of transection with varying degrees of tendon retraction. In a few cases, there was a more definable fluid signal gap at the level of release.
This study was notable for significant atrophy of the iliopsoas at the L5 pedicle and also at the level of the AIIS. At the L5 pedicle, there was nearly a 50% decrease in the cross-sectional area of the muscle. The muscle at the more proximal level also showed mild fatty infiltration in the majority of cases using the Goutallier-Fuchs classification system. Subjective assessment of the tendon in the area of release was notable for marked thickening of the tendon above and below the level of transection.
Given the amount of atrophy found in this study and the relatively poor patient-reported outcomes, as well as the iliopsoas' role as the primary hip flexor tendon, clinical weakness following release would be expected.13 However, previous studies have demonstrated no significant difference in objective seated hip flexion strength using the standard 5-point manual muscle testing scale.6,26,27 There are several limitations using this 5-point manual muscle testing scale that weaken its clinical significance; the test is subjective and relies on patient effort, and the categorical nature does not allow for detection of small differences. Brandenburg et al12 recently obtained continuous quantitative objective measurements of strength using a custom testing apparatus; they found a statistically significant reduction in seated hip flexion strength (19%) following iliopsoas release with no significant change in supine hip flexion strength.
In addition to demonstrating decreased hip flexion strength following release, Brandenburg et al12 demonstrated a reduction in the volume of the iliopsoas on the operative side compared with the nonoperative side. The findings of the current study support this as well, but preoperative and postoperative measurements of the same side were used instead of comparing only the contralateral side. However, using the contralateral side for comparison adds a possible confounder of limb dominance.28
Mean time between surgery and postoperative imaging was 581 days (range, 71 to 2076 days), and some of the measured variables might have been influenced by time and not the procedure itself. However, Pearson correlation showed no relation between time and atrophy (P=.83) or Goutallier-Fuchs score (P=.35). Similarly, Hain et al16 found no edema as early as 6 months after iliopsoas tenotomy using MRI arthrography. In addition, based on a multivariate regression analysis, the current study demonstrated no relationship between muscle area and length of time after surgery, sex, or age. The known relationship between cross-sectional area and muscle force also would suggest the strength of the iliopsoas may not improve as time progresses after surgery because the amount of atrophy does not seem to improve.29–31
The decreased area of the iliopsoas is concerning not only for hip flexion strength; Domb et al32 hypothesized it also may play a role in hip stability due to its location in the front of the hip joint. Iliopsoas snapping most commonly is associated in patients with an aspect of osseous instability.33 Persistent area decreases in the iliopsoas may lead to negative long-term effects on the stability of the hip.34 This relationship requires further research to establish an absolute relationship.
The difference in psoas area at the L5 pedicle was 44% of the area of the contralateral side compared with an area decrease of only 16% preoperatively vs postoperatively at the distal level of the AIIS. This larger difference in area more proximally simply may be due to the higher percentage of muscle present proximally; alternatively, it may be due to the anatomy of the iliopsoas and the portion of this tendon that is sectioned during release. In an MRI and cadaveric study. Polster et al35 noted the tendinous portion of the iliopsoas at the level of the hip joint is composed almost entirely of the psoas tendon, and the majority of muscle is contributed by the iliacus, especially the lateral fibers, which are not included in the tendon and insert directly onto the femur without tendinous attachment.
In this study, the patient-reported outcomes had a mean mHHS of 51. This is significantly worse than the usual patient-reported outcomes after hip arthroscopy in general (84.0), as well as after hip arthroscopy that included iliopsoas release (83.2).36 In addition, the patient acceptable symptomatic state (PASS) is a number indicating that if patients attain this number, they are generally very satisfied with the result of their hip procedure. The PASS for the mHHS is noted to be 74. These numbers indicate the patients who responded were doing poorly from a subjective standpoint. This is a confounder when analyzing iliopsoas atrophy as the atrophy also could be attributed to decreased use from pain and decreased function.
This study had multiple limitations. The most significant of these revolved around the demographic of this cohort. All of the patients in this study required additional imaging after their operation, which indicates they potentially had residual hip pathology. To evaluate the ipsilateral psoas, this was inevitable given that postoperative imaging is not routinely performed in healthy patients. This is reflected in the below-average mHHS and HOS scores reported by the patients in this study. As such, the authors were not able to establish a causative relationship between iliopsoas release and poor clinical outcomes. In addition, it is possible the atrophy is not entirely due to the release and may be a product of the poor functional outcome from surgery, which may lead to additional atrophy in this select population.
Additional limitations included the retrospective nature of this study and the weaknesses associated with any retrospective study. Not all imaging included the entire pelvis; therefore, potential proximal retraction of the tendon and muscle belly may lead to a false report of atrophy at the level of the AIIS. However, in this study, more severe atrophy was found compared with the contralateral side at the level of the L5 pedicle near the origin of the psoas, arguing against this possibility. Because the authors did not draw any conclusions from the morphological assessments and simply included the assessments as a means of confirming the results of this study, the subjectivity with which they were measured is unlikely to have a meaningful impact. Significance of the patient-reported outcomes would have benefited greatly from a comparison to baseline as well as a higher rate of follow-up. One large limitation of this study was the lack of a comparison group that did not have iliopsoas release to better discern whether the atrophy was scope- or pain-related, or truly a product of iliopsoas release. Finally, the Goutallier-Fuchs scale of muscle atrophy was designed for the shoulder and has been validated for hip abductors but not for hip flexors.21
Following release of the iliopsoas tendon, assessment with advanced imaging demonstrated significant decreases in the cross-sectional area of the muscle-tendon unit both proximally and distally with significant fat atrophy of the muscle. This likely has implications for patients' hip flexor strength as well as hip stability. These factors should be taken into account when considering release of the iliopsoas tendon during hip arthroscopy.
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