Modern implant design

Current hip resurfacing implants use a metal-on-metal bearing surface, with implants forged or cast from high-carbon cobalt-chromium-molybdenum alloy. If the femoral and acetabular implants are well-manufactured and well-positioned, large-diameter metal-on-metal bearing surfaces are predicted by lubrication theory to produce very low levels of volumetric wear compared with the metal-on-polyethylene surfaces used in the past.

Hybrid fixation (press-fit acetabular and cemented femoral components) is most commonly used. A cementless acetabular cup is typically press fit into the under-reamed acetabulum. The acetabular cup has a surface modification of cobalt-chromium beads or plasma-sprayed titanium with or without hydroxyapatite coating for bone in-growth and fixation. The femoral component is traditionally cemented into place on the femoral neck. There are, however, cementless designs being used because of the potential for thermal bone necrosis during the cement curing process, contributing to early implant failure.

Surgical technique

There is no ideal surgical approach for hip resurfacing arthroplasty. There is a balance between the risks of surgical dissection and necessary exposure and visualization. Femoral head oxygen concentration is compromised in both the anterolateral and posterior approaches. However, during the anterolateral approach and not the posterior approach, femoral oxygen concentration recovers after implantation with hip relocation ( Table 1 ), suggesting compromise of the ascending branch of the medial circumflex femoral artery during release of the short external rotators during the posterior approach. However, a posterior approach used in surgical dislocation that protects the tendon of the obturator externus and consequently the ascending branch of the medial circumflex femoral artery has no risk of femoral head osteonecrosis (see Table 1 ). Retrieval studies demonstrated osteonecrosis of the remaining femoral head in 10 of 14 failed hip resurfacing arthroplasties using the posterior approach; 9 underwent revision for femoral neck fracture, and 1 underwent revision for femoral component loosening.

Preparation of the femoral head by reaming is enough to decrease blood flow to the femoral head by 70% in 9 of 10 hips. This is likely a result of disruption of the nutrient retinacular vessels of the femoral head because 80% of these vessels penetrate bone in the anterosuperior and posterosuperior quadrants of the femoral neck. Valgus positioning during reaming results in notching of the femoral neck and a reduction of blood flow by 50% in 10 of 14 hips by laser Doppler flowmetry (see Table 1 ). Notching is associated with an increase in femoral component loosening, from 6.8% to 28.6%, during radiographic follow-up of 72 hip resurfacing arthroplasties (see Table 1 ). Removal of up to 30% of the anterolateral femoral neck does not alter the load-bearing capacity of the proximal femur, suggesting that there is minimal mechanical weakness attributed to femoral neck notching and that there is a vascular component to femoral neck notching that contributes to loosening as opposed to fracture.

The heat produced by polymerization of the cement that is used to fix the femoral component may result in thermal bone necrosis. Retrieval studies of 96 failed hip resurfacing arthroplasties demonstrate that femoral implants that failed secondary to femoral loosening had an average cement mantle of 2.9 mm, whereas nonfemoral failures had an average cement mantel of 2.3 mm. In addition, the bone-cement interface of the loose femoral components were thick, fibrous, and associated with bone resorption, and the interface of those isolated for femoral neck fracture were ischemic. Cement-filled bone cysts were more prevalent in hip resurfacing arthroplasties that failed secondary to femoral loosening. Finite element modeling supports the concept that deep cement penetration or filling of a bone cyst larger than 1 cm ³ results in temperatures that are consistent with thermal bone necrosis (see Table 1 ). Another retrieval study that evaluated 55 failed hip resurfacings supports these conclusions. Use of a 3-mm suction cannula inserted into a hole drilled in the lesser trochanter, penetrating 3 cm into the intramedullary canal for femoral preparation and cementing, resulted in a decrease in the maximal femoral temperature from 68°C to 36°C, well below the level required for thermal bone necrosis (see Table 1 ).

The positioning of the femoral component is critical. Evaluation of 94 hip resurfacing arthroplasties showed that a varus stem-shaft angle, less than 133°, correlated with adverse outcomes (see Table 1 ). Finite element analysis supports placement of the femoral implant in a relative valgus position as a result of decreased predicted peak stress levels more similar to those of a normally loaded femur ( Fig. 1 ). Valgus positioning by 10° increased load to fracture by 28%. However, radiographic studies demonstrate that placement of the femoral component in 5° to 10° of relative valgus position slightly decreases mean femoral offset by 4.5 to 8 mm and results in mean limb length discrepancy of −2.2 to 0.3 mm (see Table 1 ). Hence, hip resurfacing arthroplasty can only minimally alter hip biomechanics to correct deformity. Despite this limitation, a randomized clinical trial of 120 patients comparing THA to hip resurfacing arthroplasty confirmed a mean limb shortening of 1.9 mm and a mean reduction in horizontal offset by 3.3 mm in the hip resurfacing arthroplasty limb of the study (see Table 1 ).

Depiction of normal femoral neck shaft angle (125° between shaft and neck) as well as varus (greater than 130° between shaft and implant) and valgus (less than 130° between shaft and implant) hip resurfacing arthroplasty positioning. Note that valgus positioning aligns the femoral component with the medial cortical structures.

From Freeman MA. Some anatomic and mechanical considerations relevant to the surface replacement of the femoral head. Clin Orthop Relat Res 1978;134:19–24; with permission.

Despite issues with exposure, reaming, and cementing, the modern metal-on-metal hybrid hip resurfacing implants provide the bone stock conservation required for the implant to be viable in a younger patient population (see Table 1 ). Many acetabular components require the removal of less than 5 mm of acetabular bone stock, similar to the amount of acetabular bone stock removed during a THA. A randomized trial of 230 hips demonstrated no difference in the size of the acetabular component in hip resurfacing arthroplasty or THA. Positioning of the acetabular component is also important because an abduction angle (ie, lateral opening angle) of greater than 55° has been shown to increase wear in simulation and retrieval studies.

The conservation of bone stock on the femoral side is intuitive because a hip resurfacing arthroplasty can easily be converted to a THA in the case of complication or need for revision. There is no demonstrated loss of bone mineral density secondary to stress shielding during hip resurfacing arthroplasty in comparative studies. However, there is a 70% incidence of femoral neck narrowing seen during retrospective review of 163 hip resurfacing arthoplasties. The consequence of this finding is unknown.

Surgical technique

There is no ideal surgical approach for hip resurfacing arthroplasty. There is a balance between the risks of surgical dissection and necessary exposure and visualization. Femoral head oxygen concentration is compromised in both the anterolateral and posterior approaches. However, during the anterolateral approach and not the posterior approach, femoral oxygen concentration recovers after implantation with hip relocation ( Table 1 ), suggesting compromise of the ascending branch of the medial circumflex femoral artery during release of the short external rotators during the posterior approach. However, a posterior approach used in surgical dislocation that protects the tendon of the obturator externus and consequently the ascending branch of the medial circumflex femoral artery has no risk of femoral head osteonecrosis (see Table 1 ). Retrieval studies demonstrated osteonecrosis of the remaining femoral head in 10 of 14 failed hip resurfacing arthroplasties using the posterior approach; 9 underwent revision for femoral neck fracture, and 1 underwent revision for femoral component loosening.

Preparation of the femoral head by reaming is enough to decrease blood flow to the femoral head by 70% in 9 of 10 hips. This is likely a result of disruption of the nutrient retinacular vessels of the femoral head because 80% of these vessels penetrate bone in the anterosuperior and posterosuperior quadrants of the femoral neck. Valgus positioning during reaming results in notching of the femoral neck and a reduction of blood flow by 50% in 10 of 14 hips by laser Doppler flowmetry (see Table 1 ). Notching is associated with an increase in femoral component loosening, from 6.8% to 28.6%, during radiographic follow-up of 72 hip resurfacing arthroplasties (see Table 1 ). Removal of up to 30% of the anterolateral femoral neck does not alter the load-bearing capacity of the proximal femur, suggesting that there is minimal mechanical weakness attributed to femoral neck notching and that there is a vascular component to femoral neck notching that contributes to loosening as opposed to fracture.

The heat produced by polymerization of the cement that is used to fix the femoral component may result in thermal bone necrosis. Retrieval studies of 96 failed hip resurfacing arthroplasties demonstrate that femoral implants that failed secondary to femoral loosening had an average cement mantle of 2.9 mm, whereas nonfemoral failures had an average cement mantel of 2.3 mm. In addition, the bone-cement interface of the loose femoral components were thick, fibrous, and associated with bone resorption, and the interface of those isolated for femoral neck fracture were ischemic. Cement-filled bone cysts were more prevalent in hip resurfacing arthroplasties that failed secondary to femoral loosening. Finite element modeling supports the concept that deep cement penetration or filling of a bone cyst larger than 1 cm ³ results in temperatures that are consistent with thermal bone necrosis (see Table 1 ). Another retrieval study that evaluated 55 failed hip resurfacings supports these conclusions. Use of a 3-mm suction cannula inserted into a hole drilled in the lesser trochanter, penetrating 3 cm into the intramedullary canal for femoral preparation and cementing, resulted in a decrease in the maximal femoral temperature from 68°C to 36°C, well below the level required for thermal bone necrosis (see Table 1 ).

The positioning of the femoral component is critical. Evaluation of 94 hip resurfacing arthroplasties showed that a varus stem-shaft angle, less than 133°, correlated with adverse outcomes (see Table 1 ). Finite element analysis supports placement of the femoral implant in a relative valgus position as a result of decreased predicted peak stress levels more similar to those of a normally loaded femur ( Fig. 1 ). Valgus positioning by 10° increased load to fracture by 28%. However, radiographic studies demonstrate that placement of the femoral component in 5° to 10° of relative valgus position slightly decreases mean femoral offset by 4.5 to 8 mm and results in mean limb length discrepancy of −2.2 to 0.3 mm (see Table 1 ). Hence, hip resurfacing arthroplasty can only minimally alter hip biomechanics to correct deformity. Despite this limitation, a randomized clinical trial of 120 patients comparing THA to hip resurfacing arthroplasty confirmed a mean limb shortening of 1.9 mm and a mean reduction in horizontal offset by 3.3 mm in the hip resurfacing arthroplasty limb of the study (see Table 1 ).

Depiction of normal femoral neck shaft angle (125° between shaft and neck) as well as varus (greater than 130° between shaft and implant) and valgus (less than 130° between shaft and implant) hip resurfacing arthroplasty positioning. Note that valgus positioning aligns the femoral component with the medial cortical structures.

From Freeman MA. Some anatomic and mechanical considerations relevant to the surface replacement of the femoral head. Clin Orthop Relat Res 1978;134:19–24; with permission.

Despite issues with exposure, reaming, and cementing, the modern metal-on-metal hybrid hip resurfacing implants provide the bone stock conservation required for the implant to be viable in a younger patient population (see Table 1 ). Many acetabular components require the removal of less than 5 mm of acetabular bone stock, similar to the amount of acetabular bone stock removed during a THA. A randomized trial of 230 hips demonstrated no difference in the size of the acetabular component in hip resurfacing arthroplasty or THA. Positioning of the acetabular component is also important because an abduction angle (ie, lateral opening angle) of greater than 55° has been shown to increase wear in simulation and retrieval studies.

The conservation of bone stock on the femoral side is intuitive because a hip resurfacing arthroplasty can easily be converted to a THA in the case of complication or need for revision. There is no demonstrated loss of bone mineral density secondary to stress shielding during hip resurfacing arthroplasty in comparative studies. However, there is a 70% incidence of femoral neck narrowing seen during retrospective review of 163 hip resurfacing arthoplasties. The consequence of this finding is unknown.