Femoral Neck Notching in Dual Mobility Implants: Is This a Reason for Concern?

BACKGROUND: Dual mobility (DM) total hip arthroplasty (THA) implants have been advocated for patients at risk for impingement due to abnormal spinopelvic mobility. Impingement against cobalt-chromium acetabular bearings, however, can result in notching of titanium femoral stems. This study investigated the incidence of femoral stem notching associated with DM implants and sought to identify risk factors. METHODS: A multicenter retrospective study reviewed 256 modular and 32 monoblock DM components with minimum 1-year clinical and radiographic follow-up, including 112 revisions, 4 conversion THAs, and 172 primary THAs. Radiographs were inspected for evidence of femoral notching and to calculate acetabular inclination and anteversion. Revisions and dislocations were recorded. RESULTS: Ten cases of femoral notching were discovered (3.5%), all associated with modular cylindrospheric cobalt-chromium DM implants (P = .049). Notches were first observed radiographically at mean 1.3 years after surgery (range 0.5-2.7 years). Notch location was anterior (20%), superior (60%), or posterior (20%) on the prosthetic femoral neck. Notch depth ranged from 1.7% to 20% of the prosthetic neck diameter. Eight cases with notching had lumbar pathology that can affect spinopelvic mobility. None of these notches resulted in stem fracture, at mean 2.7-year follow-up (range 1-7.6 years). There were no dislocations or revisions in patients with notching. CONCLUSION: Femoral notching was identified in 3.5% of DM cases, slightly surpassing the dislocation rate in a cohort selected for risk of impingement and instability. Although these cases of notching have not resulted in catastrophic failures thus far, further study of clinical sequelae is warranted. Component position, spinopelvic mobility, and implant design may influence risk.

New findings in hip capsular anatomy: dimensions of capsular thickness and pericapsular contributions.

PURPOSE: The purpose of this investigation was to provide a detailed description of the anatomy of the hip capsule and pericapsular structures. METHODS: Dissections were performed on 11 nonpaired, fresh-frozen cadaveric hips by 2 independent observers: 1 fellowship-trained orthopaedic total joint surgeon and 1 chief orthopaedic surgery resident. Documentation of capsular thickness, origins, insertions, and attachments to pericapsular structures including the abductors, rectus femoris, piriformis, short external rotators, and iliocapsularis muscles was performed. Tendinous insertions of the surrounding pericapsular muscles were measured according to size and distance from reproducible osseous landmarks. RESULTS: The capsule is thickest near the acetabular origin at the posterosuperior and superior hemi-quadrants and is thinnest near the femoral insertion in the posterior and posteroinferior hemi-quadrants. The iliocapsularis, indirect head of the rectus, conjoint, obturator externus, and gluteus minimus tendons all show consistent capsular contributions, whereas the piriformis does not have a capsular attachment. Osseous landmarks for tendinous attachments are defined and illustrated. The inter-relation of these structures is complex, yet their relations to the anterior hip capsule and contributions to its thickness are predictable. CONCLUSIONS: The dynamic pericapsular structures pertinent to the hip arthroscopist include the iliocapsularis, gluteus minimus, and reflected head of the rectus femoris. At the acetabulum, the thickest region of the capsule is posterosuperior and superolateral. At the femoral insertion, the thickest region is anterior. CLINICAL RELEVANCE: Knowledge of the intricate relation between the hip capsule and pericapsular structures presented here will be useful for surgeons as they perform the precise and specific capsular releases required during hip arthroscopy. Our anatomic findings contribute important qualitative data that build on the recent literature regarding the importance of capsular management during hip arthroscopy to postoperative hip stability.