Incidence and Significance of Femoral Neck Narrowing in the First 500 Conserve® Plus Series of Hip Resurfacing Cases: A Clinical and Histologic Study

Narrowing of the femoral neck after metal-on-metal hip resurfacing arthoplasty has been reported as a common radiologic feature, although its significance is still unknown. This study reports the presence and significance of neck narrowing in the first 500 consecutive Conserve® Plus metal-on-metal hip resurfacings in 431 patients.

Metal-on-metal hip resurfacing is an attractive option for young patients, as it leaves more femoral bone stock if revision surgery is necessary. It also restores normal anatomy and biomechanics of the hip, while providing near-normal proximal femoral anatomy and loading. Narrowing or thinning of the femoral neck after metal-on-metal hip resurfacing arthroplasty has been reported as a common radiologic feature. Although the long-term effects of neck narrowing are unknown, it has been speculated that neck narrowing does not result in adverse clinical or radiologic outcomes.

This study reports the incidence and significance of neck narrowing in the first 500 consecutive hips in the senior surgeon’s (H.C.A.) series with a relatively long follow-up and determines the cause where possible. The radiological features of neck narrowing were examined over time, as well as the clinical features and histology of retrievals of those hips that failed with neck narrowing.

Materials and methods

Between 1996 and 2002, the senior author (H.C.A.) implanted the first 500 consecutive Conserve® Plus metal-on-metal hip resurfacings (Wright Medical Technology, Inc, Arlington, TN, USA) in 431 patients (319 men, 112 women). The mean age at the time of surgery was 48.6 years, and the average follow-up was 95.9 (range, 1.4–161) months. Two patients were lost to follow-up. Neck narrowing was measured using a modified protocol used by Hing and colleagues. Immediate postoperative radiographs were compared with the most recent postoperative radiographs for narrowing of the femoral neck at the component-neck junction ( Fig. 1 ). All radiographs used were standardized anteroposterior (AP) views. Nonsuitable radiographs were excluded from the measurement (eg, femoral orientation not consistent with the rest of the series). Radiographic measurements of acetabular abduction and anteversion were made using the EBRA program (Einzel-Bild-Roentgen-Analysis, University of Innsbruck, Austria), and this program was also used to outline the femoral ball to find the center of the femoral head. Using the outline and the center of the femoral head, the Image J software (National Institute of Health, version 1.41) was used to measure the femoral ball diameter and the diameter of the neck at the component-neck junction. This procedure was performed by 2 independent observers (J.Y., K.M.T.). Radiographs that showed narrowing were evaluated at different time points over the series of follow-up visits to track changes in narrowing. The neck narrowing measurements were plotted against time to examine the progression of narrowing over the follow-up period. The femoral head diameter was used to calibrate the radiographs to measure the amount of neck narrowing in millimeters and to calculate the percentage of narrowing. Cases exceeding 10% narrowing were further classified depending on whether the narrowing involved most of the neck or was caused by the presence of a more localized triangular area or “bite,” with loss of bone at the component-neck junction and sclerotic line at the bite ( Fig. 2 , Table 1 ). Furthermore, the narrowing was assessed as either symmetric (both inferior and superior narrowing) or eccentric (mostly inferior or mostly superior narrowing). This assessment was done by the senior author (H.C.A.).

Table 1

Neck narrowing classification

Category	Description
I	Neck narrowing with a bite at the component-neck junction
II	Neck narrowing involving most of the neck

After the initial analysis of neck narrowing, we performed a secondary biomechanical analysis similar to that of Joseph and colleagues to measure the abductor moment arm (AMA) and body moment arm (BMA) in 301 preoperative and 478 postoperative radiographs to calculate the hip ratio (HR), which indicates the force exerted by the abductor muscles and body weight to maintain equilibrium in the 1-leg stance ( Fig. 3 ). In addition, the stem shaft angle (SSA) was calculated for each hip. Radiographs without the entire AP view of the pelvis or with incomplete view of the greater trochanter were excluded from the biomechanical analysis. For the preoperative biomechanical analysis, 279 radiographs were available for the non–neck narrowing group and 22 for the neck narrowing group. For the postoperative biomechanical analysis, 446 radiographs were available for the non–neck narrowing group and 25 were available for the neck narrowing group.

The difference in preoperative and postoperative HR was calculated to assess changes in hip biomechanics. Clinical and radiologic variables including the underlying diagnosis, biomechanical and radiographic parameters (neck narrowing, abduction, anteversion, contact-patch-to-rim [CPR] distance as described by Langton and colleagues, SSA, preoperative and postoperative HR, change in HR, postoperative AMA, postoperative BMA), demographic data (age, height, weight, body mass index [BMI], femoral head size), and patient assessment scores (Harris hip score [HHS], University of California Los Angeles [UCLA] activity score) were compared between the neck narrowing and non–neck narrowing groups.

Two-tailed Student t -tests were used to compare parametric data. Mann-Whitney-U tests were used to compare nonparametric data. A P value of less than 0.05 was deemed to be significant. χ ²Analysis with Fisher exact test correction was used to compare the ratio of failure with nonfailure cases between the groups with and without neck narrowing.

Retrieved implants were examined at the Implant Retrieval Laboratory at the Los Angeles Orthopaedic Hospital. Upon retrieval, the specimens were fixed in formalin. The retrieved implants were subsequently cleaned, photographed, and examined grossly. Then the femoral component and associated femoral neck were sectioned using an EXAKT saw (EXAKT Advanced Technologies, Norderstedt, Germany) into 2.5-mm slices, which were then radiographed. These slices were removed from the metal, decalcified, and processed into paraffin for routine histologic examination and staining by hematoxylin-eosin. The cut sections that included the femoral neck, neck implant junction, and resurfaced bone portions were examined for osteoclastic and osteoblastic activity, as well as viability of the bone, which was assessed by the presence of nuclei in the bone. Soft tissues adhering to the neck and interfacial membranes were examined for the presence of inflammatory cells and wear debris.

Results

Twenty-five hips in 22 patients were identified in the first 500 hips to have neck narrowing greater than 10%, which gives an incidence of 5%. In hips with neck narrowing, there were 11 males (13 hips, 52%) and 11 females (12 hips, 48%). In hips without neck narrowing, there were 311 males (358 hips, 77.2%) and 106 females (117 hips, 25.2%). There was a significant difference in gender between the two groups (Chi-squared, P = .001), with a higher proportion of females in the neck narrowing group, occurring in 3.6% of male hips and 10.3% female hips.

There was no difference in the underlying diagnosis among cases with neck narrowing compared with the rest of the cohort (χ ², P = .36) ( Table 2 ). There was 20.1% (10.8%–38.7%) narrowing in the neck narrowing group, and the narrowing measured 7.1 mm (3.5–14.2 mm) at the last follow-up (mean, 101 months).

Table 2

Etiology

Etiology	No Neck Narrowing	Neck Narrowing	Total
OA	300 (63.2%)	15 (60%)	315 (63%)
ON	39 (8.2%)	2 (8%)	41 (8.2%)
DDH	55 (11.6%)	2 (8%)	57 (11.4%)
Posttrauma	38 (8%)	1 (4%)	39 (7.8%)
Inflammatory
Inflammatory OA	9 (1.9%)	—	9 (1.8%)
Ankylosing Spondylitis	3 (0.6%)	1 (4.0%)	4 (0.8%)
JRA	3 (0.6%)	—	3 (0.6%)
RA	4 (0.8%)	—	4 (0.8%)
RD	1 (0.2%)	—	1 (0.2%)
Childhood disorders
LCP	12 (2.5%)	1 (4%)	13 (2.6%)
SCFE	7 (1.5%)	2 (8%)	9 (1.8%)
Other
Melorheostosis	1 (0.2%)	—	1 (0.2%)
Epiphyseal Dysplasia	3 (0.6%)	1 (4%)	4 (0.8%)
Total	475	25	500