Summary

Range of hip motion may help predict if a patient has hip microinstability as compared with FAI.

Abstract

Objective

Clues to the diagnosis of hip microinstability is still evolving. We hypothesized that hip range of motion (ROM) will be greater in patients with hip microinstability than in patients with femoroacetabular impingement (FAI) alone.

Methods

After IRB approval, a retrospective chart review was performed for patients undergoing arthroscopic surgery for hip microinstability (thus arthroscopically confirmed microinstability) and FAI at an academic center. A power analysis determined 25 patients were needed in each group to detect a 5 degree difference. After adjusting for gender, BMI, age, and smoking status, multivariable generalized linear models were used to detect differences. Sensitivity and specificity were determined for multiple flexion plus rotation arc measures. Finally, a logistic regression was used to create a model to determine the probability of microinstability with a given gender, BMI, age and degree of flexion. Microinstability was defined as any patient requiring capsular plication for instability based on previously published intra-operative parameters at our institution. FAI was defined as alpha angle >55 degrees, center edge angle >35 degrees, or other radiographic parameter of pincer impingement, such as crossing sign. ROM including hip flexion, internal rotation, external rotation and rotation arc were recorded in degrees.

Results

Seventy five patients were included in this study: 25 with isolated hip microinstability, 25 with isolated FAI, and 25 patients had combined microinstability with FAI (CM-FAI). No difference in ROM was found between microinstability and CM-FAI groups for all ROM parameters. However, the isolated microinstability and CM-FAI groups had significantly greater hip flexion (128°, 135°) than FAI only (113°, p=0.003, p<0.001). Isolated hip microinstability and CM-FAI groups also had significantly greater flexion plus rotation (218°, 224°), than FAI only (180°, p=0.015, p=0.004). Flexion plus internal rotation was greater in the isolated microinstability and CM-FAI groups (162°, 168°) than in FAI only (131°, p=0.005, p<0.001). CM-FAI had a greater flexion plus external rotation (190°) than FAI only (162°, p=0.012). Rotational parameters alone showed no difference between instability and FAI groups in isolation. When flexion plus rotation arc was greater than 197.5 degrees, there was an 84% sensitivity and 76% specificity that a component of instability present. Furthermore, a risk of instability profile may be built using hip flexion, BMI, age and gender determine the probability of instability.

Conclusions

Hip microinstability is associated with a significant increase in multiple ROM parameters, even in the presence of FAI. Flexion was the most important parameter. An in-office calculation of flexion plus rotation arc greater than 197.5 degrees can be used to suggest the potential of hip microinstability as a component hip pain. Finally, flexion, BMI, age, and gender can be combined to determine the probability of hip microinstability and potentially predict the need for capsular plication.