Femoroacetabular impingement (FAI) has been identified as the cause of idiopathic osteoarthritis in young patients. FAI is the result of decreased femoral head/neck offset ratio due to bony deformities and causes hip pain and labral tears. Because the unique design and bone preserving nature of metal-onmetal hip resurfacing implants, it is extremely difficult to correct extensive bony deformities associated with FAI. Poor patient selection and lack of orrection/undercorrection of the underlying FAI deformity may lead to prosthetic impingement, extensive wear and metal ion release, component loosening, and subsequent implant failure. Hence, it is critical to define the patient population undergoing hip resurfacing. Because metal-on-metal hip resurfacing is performed more frequently in a younger population, we hypothesize that this patient population will have a larger proportion of femoroacetabular impingement than the general population and identification of this patient population is critical to the longevity of the implant.

A retrospective review of 153 hips undergoing metal-on-metal hip resurfacing was performed. 52 hips were excluded based on the exclusion criteria of inadequate preoperative films (6 subjects), existing hardware/history of trauma (11 subjects), or if the resurfacing was performed due to avascular necrosis secondary to trauma, steroids, etc (35 subjects). The remaining 101 hips (76 male, 25 female) had an average age of 51.8 years. Preoperative x-rays were utilized to assess impingement according to previously published methods. An acetabular index (AI) of x ≤ 0°, center edge angle (CE angle) of x > 39°, a Sharp angle of x < 33°, and a present cross-over sign were considered pathologic findings for pincer impingement. Pathologic findings for cam impingement included the triangular index (TI; pathologic with R=r+2mm) and an α angle greater than 83° in men or 57° in women. Subjects were categorized as having impingement if they had one or more pathologic finding for either cam or pincer impingement and as having mixed impingement if they had one pathologic finding for both cam and pincer measurements. Prevalence rates were compared to published data for the general population.

Fifty-five subjects had at least one pathologic finding for cam impingement (18, 7, and 30 subjects had pathologic measurements for α angle, TI, and both measurements, respectively); 24 subjects had at least one pathologic pincer measurement (4, 6, 14, and 4 pathologic measurements for AI, CE angle, cross-over sign, and Sharp angle, respectively; 3 subjects had multiple pincer findings) 13 subjects were classified as having mixed impingement (with α angle and cross-over sign as the most prevalent cam and pincer measurements). When compared to published data for the general population (M: 17%, F: 4%), we found a significantly larger proportion of cam impingement in both males (60.5%) and females (36%) in patients undergoing resurfacing at our institution (p< 0.001). There was also a significantly larger proportion of pincer impingement in our population (23.8%) than in the general population (10.7%) (p=0.01). There was no significant difference between our proportion of patients with mixed impingement (12.9%) and the general population (20.8%) (p=0.150).

The patient population for metal-on-metal hip resurfacing shows a greater prevalance of FAI than the general population. Because the femoral head/neck junction is preserved with hip resurfacing, patients undergoing this type of procedure might be at increased risk of impingement. Hence, it is important to assess the degree of FAI preoperatively. This will allow proper patient selection and careful planning of surgical correction of the underlying FAI deformity to increase implant longevity.

Hip resurfacing arthroplasty is a technically challenging procedure, and orientation of the femoral component is critical to avoid implant failure. Recently, numerous articles have shown that the use of computer-assisted navigation decreases the learning curve for beginners in hip resurfacing and to improve the surgeon’s ability to produce consistent results. The purpose of this study was to evaluate the learning curve of computer-assisted navigation in the hands of an experienced hip resurfacing surgeon.

This retrospective study was compromised of 100 metal-on-metal total hip resurfacings in 94 patients. The resurfacings were performed by a single fellowship-trained surgeon, with hip resurfacing experience of more than 250 hip resurfacings without navigation. Data collected included gender, age at the time of surgery, BMI, operative time, postoperative complications, and digital planning. Standard nteroposterior (AP) radiographs taken in the preoperative and postoperative period were evaluated to measure neck-shaft and stem-shaft angles, respectively. There were 24 females and 70 males, who had a mean age of 49 years (range, 19 to 68 years). The 100 hips were arranged chronologically by operative date and broken down into four groups of 25. Data also was gathered on 25 non-navigated hip resurfacings to serve as a matching group.

There were no significant differences found between the four groups and matching groups with respect to patient variables, including age, BMI, or gender. There were also no significant differences found among the groups with respect to OR time (p = 0.565). The mean OR time for all 100 navigated hips was 101 minutes, compared to a mean of 104 minutes for the matching group (p = 0.924). Using linear regression analysis, the only variable that was found to influence OR time was BMI (p < 0.001). The mean actual stem-shaft angle (SSA) of the groups became more valgus over time, with group 1 having an SSA of 139; group 2, an SSA of 140; group 3, an SSA of 142; and group 4, an SSA of 144. Compared to the preoperative neck-shaft angle, the postoperative stem-shaft angle for 89% of the femoral components was inserted in a valgus position, with 96% of those in group 4 being inserted in a valgus position. The matching non-navigated group had only 80% of the cases with the stem-shaft angle inserted in valgus.

The data presented here demonstrates that providing an imageless computer-assisted navigation system to an experienced hip resurfacing surgeon offered the benefits of navigated surgery including increased accuracy, with no learning curve effect. Computer-assisted navigation can help the learning curve of a technically demanding procedure in inexperienced surgeons, as described by the literature, while placing real-time feedback and consistent repeatability into the hands of an experienced surgeon.