Comparison of Fully Porous-Coated and Hybrid Hip Resurfacing: A Minimum 2-Year Follow-Up Study

The purpose of this study was to compare clinical and radiological outcomes of the first 191 fully porous-coated hip resurfacing arthroplasties with 96 hybrid hip resurfacing arthroplasties performed during the same period at a minimum 2-year follow-up to evaluate the initial fixation of uncemented femoral resurfacing components. The results of this study indicate that fully porous-coated femoral resurfacing components can routinely achieve reliable fixation and provide similar initial results as have been achieved with cemented fixation. Long-term results are needed to determine which type of fixation is superior for the femoral hip resurfacing component.

Surgeons have debated the relative value of cemented versus uncemented fixation in total hip arthroplasty (THA) for the last few decades. Because orthopedic surgeons have come to a general consensus on the superiority of uncemented fixation, uncemented fixation has virtually replaced cemented fixation in stemmed THAs in the United States. Most acetabular components that are used today are of the uncemented type, as are about 80% of femoral stems. This trend is occurring in Canada and Europe as well.

Hip resurfacing arthroplasty (HRA) has become an accepted alternative to stemmed THA in the last decade for young active patients wishing to return to a high level of activity, including impact sports. In modern metal-on-metal HRAs, hybrid fixation (uncemented acetabular and cemented femoral components) has been adopted as the standard. Many studies have demonstrated successful early and midterm outcomes with survivorship rates of up to 97%. In contemporary HRAs, cement technique has been shown to be quite variable and difficult to control. Increasing cement thickness may cause more thermal necrosis and a higher risk of stress shielding. Excess cement under the femoral component may result in leaving the femoral component incompletely seated; this extends the femoral neck, leaving uncovered reamed bone, and increases the risk of fracture. Also, the mechanical properties of the cement may result in an increasing rate of aseptic loosening over time. Our results at the midterm (average 5.6 years) follow-up of the Corin (Corin, Cirencester, Gloucestershire, UK) Food and Drug Administration Investigational Device Exemption study of hybrid HRAs have demonstrated that cement fixation failure is the most common source of failure at a rate of 3%, accounting for half of all failures.

Because of the superior results of bone ingrowth implants in stemmed THA, it may be logical to also apply uncemented fixation techniques to femoral resurfacing components. Several studies have reported positive clinical outcomes of contemporary metal-on-metal HRAs using nonporous uncemented fixation methods on both the femoral and the acetabular side. We hypothesized that the clinical results of bone ingrowth type uncemented fixation on the femoral side in HRA should be equal to, if not superior to, cemented fixation at early follow-up. The purpose of this study was to report our 2-year clinical and radiological follow-up of the first modern fully porous-coated (uncemented) hip resurfacing prosthesis and to compare these results with those of hybrid hip resurfacing prostheses manufactured by the same company.

Material and methods

The senior author (T.P.G) implanted the first fully porous-coated metal-on-metal Recap and Magnum combination hip resurfacing prosthesis (Biomet, Warsaw, IN, USA) in March 2007 ( Fig. 1 ). The Biomet uncemented femoral hip resurfacing component has a full coating of titanium plasma spray under the entire undersurface of the femoral component, excluding the stem. The porous coating is identical to that present on the Magnum acetabular component and numerous other Biomet products. There are no other changes from the cemented version of the Recap device. The cemented Recap femoral component, used in the control group, has a grit blast cobalt chrome surface. The same instrumentation was used for both versions, which is designed to allow a 0.5-mm gap for cement and a press fit for the uncemented version. The Magnum acetabular component, used in both groups, has a full porous plasma spray titanium coating and 4 small radial splines. These implants and their instrumentation were described in detail in a previous publication. Use of either the cemented or fully porous-coated Recap femoral component together with the Magnum acetabular component for an HRA procedure constitutes an off-label use in the United States; however, these implants are approved for total hip resurfacing in most other countries. Consent was obtained from each patient.

For the first year (from March 2007–February 2008), the manufacturer could not keep pace with the surgeon’s demand for the fully porous-coated femoral implant. Implant type used in each individual case was therefore determined by the availability of the uncemented implant at the time of surgery. The decision regarding fixation type was made during the operation at the time of femoral preparation. If an uncemented femoral component of the correct size was available, it was used; otherwise, the cemented version was used. There was no supply limit of the cemented version. All the fully porous-coated and hybrid HRAs performed by the senior author during this period of time were included in this study. The surgical technique was the same for both groups, with exception of the fixation method on the femoral side.

All patients younger than 65 years who were otherwise candidates for THA and who had less than one-third of their femoral head bone stock missing and who had enough acetabular wall integrity to fix a screwless acetabular component were offered HRA (occasionally, older patients were accommodated if they insisted on HRA). It is not our practice to select patients for HRA based on implant size, gender, diagnosis, and presence of cysts or deformity.

There are no additional selection criteria for the type of femoral fixation ( Table 1 ). No cases of uncemented femoral fixation were abandoned because of a failure to achieve adequate initial fixation or for any other reason. All cases had a tight initial press fit by manual testing, and none failed to seat at the desired level (within 1–2 mm of the trial position). In the cemented components, only the undersurfaces of the component, but not the stems, were cemented. In both groups, cysts were treated in the same manner: grafting with platelet concentrate and acetabular reamings. No attempts were made to remove any femoral component (either type) after it was implanted. No patients scheduled for HRA were converted to THA intraoperatively for any reason.

Table 1

Summary of demographic and diagnosis data between fully porous-coated and hybrid HRAs

	Fully Porous-Coated	Hybrid	P Value
Surgical Date	March 2007–February 2008	March 2007–February 2008	—
Number of hips	191	96	—
Number of patients	182	90	—
Age at surgery (y)	50 ± 8 (range: 28–73)	52 ± 8 (range: 21–68)	.08
Weight (kg)	84.6 ± 16.2 (range: 49.50–132.75)	84.6 ± 18.0 (range: 51.3–167.5)	.93
Body mass index	27 ± 4 (range: 19–43)	28 ± 4 (range: 18–43)	.48
T score (Bone mineral density)	0.3 ± 1.6 (range: −2.2–5.2)	0 ± 1.19 (range: −2.1–2.8)	.10
Gender			.06
Women	50 (27%)	35 (39%)	—
Men	132 (73%)	55 (61%)	—
Side			.32
Left	92 (48%)	53 (55%)	—
Right	99 (52%)	43 (45%)	—
Diagnosis			.46
Osteoarthritis	133 (70%)	74 (77%)	—
Dysplasia	31 (16%)	14 (15%)	—
Posttrauma	8 (4%)	1 (1%)	—
Avascular Necrosis	12 (6%)	4 (4%)	—
Others	7 (4%)	3 (3%)	—