The results of metal-on-metal hip Conserve® Plus resurfacings with up to 14 years of follow-up with and without risk factors of small component size and/or large femoral defects were compared as performed with either first- or second-generation surgical techniques. There was a 99.7% survivorship at ten years for ideal hips (large components and small defects) and a 95.3% survivorship for hips with risk factors optimized technique has measurably improved durability in patients with risk factors at the 8-year mark. The lessons learned can help offset the observed learning curve of resurfacing.
Metal-on-metal hip resurfacing (MMHR) is an attractive alternative to total hip replacement (THR); it provides a more anatomic and physiologic replacement, preserves more of the femoral bone, has lower dislocation rates, and has increased activity levels. Although at short-term follow-up MMHRs have comparable results to THR there are unique risks associated with resurfacing, such as femoral neck fracture. At short- or midterm follow-up, many studies have cited the importance of patient selection to prevent complications and optimize durability. Early survivorship results (>5 years) range between 93% and 99.14% and midterm results (between 5 to 10 years) between 91.5% and 95.7%. The long-term performance of metal-on-metal resurfacing implants has been shown affected by component size and the presence of large defects in the femoral head. Femoral size was found the best predictor of revision when all covariates were analyzed in birmingham hip resurfacings (BHR) in a study by McBryde and colleagues, a result corroborated by the annual report of the Australian Orthopaedic Association National Joint Replacement Registry. More recently, survivorship of hip resurfacings has been adversely affected by adverse local tissue reactions due to wear, a complication also related in part to small prosthetic size.
The purpose of the authors’ study was twofold: (1) to compare the long-term survivorship and clinical results between the hips with a femoral component size greater than 46 mm and head defects less than 1 cm (ideal hips) and those with at least one of the risk factors of femoral component size less than 46 mm and femoral head defect greater than 1 cm and (2) to assess the effects of a modified surgical technique on the survivorship of both ideal hips and hips with risk factors.
Materials and methods
Between 1996 and 2008, the senior surgeon (HCA) implanted 1100 Conserve® Plus MMHR (Wright Medical Technology, Arlington, Tennessee) devices in 924 patients using the cementing technique recommended for the Conserve® Plus femoral component, leaving a l-mm cement mantle. The device is manufactured as cast cobalt-chrome material, which is heat treated, solution annealed, machined, and polished. The senior surgeon made several changes to the surgical technique, which happened over time, and second-generation changes were complete by hip #300 ( Table 1 ). Those changes were detailed in a previous study that compared the first-generation results with 6.8 years’ follow-up to the second-generation results with 4.5 years of follow-up. In previous reports based on a subsection of the present series, the independent effects of component size and presence of large femoral defects and body mass index (BMI) on the survivorship of the procedure were highlighted. Although component size and femoral head defects were associated with clear cutoff marks usable for hip selection, the effect of BMI was linear rather than dichotomous and did not allow suggesting value for patient selection. In addition, BMI is a variable that is associated with the patient and not the hip to be treated. Venn diagrams showing the number of hips with risk factors and the number of femoral revisions in the first and second generations show that all revisions are contained within the areas covered by small component size and large femoral defects ( Figs. 1 and 2 ). For these reasons, the authors elected to constitute the groups studied in the present study on the basis of component size and presence of femoral defects only, although a BMI of less than 25 in combination with other risk factors substantially affects the incidence of failure. Consequently, the authors’ study group was composed of 468 hips in 413 patients, selected because their femoral head size was greater than 46 mm and they presented femoral head defects smaller than 1 cm. Femoral defect size was measured from intraoperativephotos, as described in a previous publication. In this group, there were 404 male (97.8%) and 9 (1.3%) female patients. The mean age of the patients was 52.1 years (range 25.4–77.5). Of these patients, 109 patients had the contralateral side resurfaced but 54 of those hips did not meet the inclusion criteria for the ideal group. All acetabular components were uncemented and all the femoral components were cemented but, in addition, the short metaphyseal stem of the femoral component was cemented in 139 out of 468 hips. In the ideal group, 205 hips were implanted with the original 5-mm shell and 263 hips with the thin 3.5-mm shell, which was introduced in October 2003 and has been used in 97% of hips since that time, enabling a larger femoral and/or a smaller socket to be inserted. The two shells were designed with identical bearing surfaces and their mechanical properties are comparable.
|First Generation||Second Generation|
|Suction||No suction (first 100 hips)||Dome suction|
|Drilled holes||A few dome holes (0 if good bone quality)—none in the chamfered area||Increased number—chamfer holes added|
|Stem cementation||Stem not cemented (only in rare cases with bad bone quality)||Stem cemented in 152 regardless of cyst size|
|Target stem shaft angle||Anatomic (first 100 hips)||140°|
|Removal of cystic debris||Incomplete—curette only||Complete—high-speed burr|