Hip Resurfacing: Preserving Vital Bone Around the Femoral Neck

Comprehensive Introduction and Patho-Epidemiology

Surface replacement represents a profoundly significant development in the continuous evolution of hip arthroplasty, fundamentally altering the reconstructive paradigm for a specific subset of patients. Hip resurfacing is predicated on the foundational premise that vital femoral bone can and should be preserved, offering a bone-conserving alternative to traditional stemmed total hip arthroplasty (THA). In this procedure, the diseased femoral head is meticulously machined and "resurfaced" through the insertion of a precisely engineered, cemented metallic component, while the acetabulum is addressed with an uncemented, press-fit cup. This anatomical reconstruction aims to restore the native biomechanics of the hip joint, preserving the femoral neck and the medullary canal for future reconstructive efforts, which is particularly advantageous given the inevitable need for revision surgery in younger demographics.

Historically, the journey of hip resurfacing has been fraught with both monumental breakthroughs and catastrophic failures. Early iterations, such as hemiresurfacing—in which only the femoral head was resurfaced against a native acetabulum—have been largely abandoned due to suboptimal clinical outcomes, rapid acetabular cartilage wear, and progressive pain. The modern era of hip resurfacing, recently approved by the U.S. Food and Drug Administration (FDA), relies heavily on metal-on-metal (MoM) tribology. Several hip resurfacing devices of various designs are currently available on the global market, including the Birmingham Hip Resurfacing (BHR), Conserve Plus, Cormet, Articular Surface Replacement (ASR), and Durom systems. These devices differ significantly in terms of metallurgy (as-cast versus forged), surface treatments (hydroxyapatite or porous titanium), cup design (hemispherical versus truncated), manufacturing processes, carbide content, component thickness, diametral clearance, and the size ranges offered.

The patho-epidemiology of coxarthrosis in the young, active patient necessitates a nuanced approach to joint reconstruction. Patients presenting with unambiguous, end-stage degenerative joint disease often possess a life expectancy that far exceeds the survivorship of standard THA implants. The mechanical demands placed on the prosthesis by high-impact activities accelerate polyethylene wear and subsequent osteolysis in traditional constructs. Hip resurfacing circumvents this by utilizing large-diameter MoM bearings that provide exceptional stability, virtually eliminating the risk of dislocation while accommodating a supra-physiologic range of motion. However, the most critical factor dictating the success of hip resurfacing is undoubtedly the surgeon’s level of expertise; the procedure is notoriously unforgiving, demanding absolute precision in component sizing and spatial orientation to prevent catastrophic early failure.

Despite the theoretical advantages, the widespread adoption of hip resurfacing has been tempered by the recognition of adverse local tissue reactions (ALTR) secondary to metal wear debris. The intricate interplay between implant design—specifically radial clearance and carbide distribution—and surgical positioning dictates the tribological success of the bearing. Edge loading, resulting from excessive acetabular cup abduction or version, exponentially increases wear rates, leading to metallosis and pseudotumor formation. Consequently, the modern orthopedic surgeon must possess a profound understanding of both the biomechanical principles governing these devices and the specific patho-epidemiological profiles of the patients in whom they are implanted.

Detailed Surgical Anatomy and Biomechanics

A comprehensive mastery of the surgical anatomy of the proximal femur and the acetabulum is paramount for the successful execution of hip resurfacing. Unlike traditional THA, where the femoral neck is resected, resurfacing mandates the absolute preservation of the femoral neck and its tenuous blood supply. The primary vascular contribution to the femoral head and neck is derived from the medial circumflex femoral artery (MCFA), which gives rise to the critical retinacular vessels. These vessels traverse the posterosuperior aspect of the femoral neck within the synovial folds. During the surgical approach and subsequent dislocation of the hip, meticulous care must be taken to preserve the joint capsule and synovial membrane around the femoral neck as much as possible. Iatrogenic disruption of this vascular network can precipitate delayed avascular necrosis (AVN) of the remaining femoral head within the implant, leading to catastrophic structural collapse and component loosening.

The biomechanical milieu of the resurfaced hip differs vastly from that of a stemmed THA. In a native hip, compressive forces are transmitted through the primary compressive trabeculae of the femoral head into the medial calcar, while tensile forces are absorbed by the lateral trabecular system. Resurfacing aims to replicate this physiologic load transfer. By capping the prepared femoral head with a metallic prosthesis, stress shielding of the proximal femur—a ubiquitous phenomenon in diaphyseal or metaphyseal engaging stems—is largely mitigated. The preservation of the proximal femoral bone stock maintains the native elastic modulus of the bone-implant construct, thereby reducing proximal femoral osteopenia and preserving vital bone for potential future revisions.

However, the biomechanics of the head-neck junction post-resurfacing present a unique vulnerability. The transition zone between the rigid metallic component and the elastic femoral neck is subjected to immense shear and bending moments. In general, a slight valgus positioning of the femoral implant is strongly recommended. Valgus alignment effectively converts deleterious shear stresses across the head-neck junction into more favorable compressive forces, significantly reducing the risk of postoperative femoral neck fracture. Conversely, varus positioning increases the bending moment arm, predisposing the construct to fatigue failure. Furthermore, the surgeon must be acutely aware of the risk of intraoperative neck notching during femoral cutting, which often occurs as a consequence of attempting an extreme valgus position. Notching creates a severe stress riser in the superior tension band of the femoral neck, drastically increasing fracture probability.

On the acetabular side, the anatomy dictates the use of large-diameter bearings, which inherently alter the tribology and stability of the joint. The large femoral head size closely approximates native anatomy, maximizing the "jump distance"—the translation required for the femoral head to dislocate from the acetabulum. This biomechanical advantage allows patients to safely return to extreme ranges of motion and high-impact sports without the looming threat of instability. However, the large diameter also increases the frictional torque at the bearing interface. If the acetabular component is malpositioned, particularly in excessive inclination or anteversion, the fluid film lubrication is disrupted. This edge-loading scenario dramatically elevates volumetric wear, releasing nanometer-sized cobalt and chromium ions into the surrounding tissues and systemic circulation, underscoring the critical need for precise anatomical restoration.

Exhaustive Indications and Contraindications

Patient selection is the single most critical determinant of long-term survivorship in hip resurfacing arthroplasty. The procedure is optimally utilized in younger, highly active patients—typically males under the age of 65 and females under the age of 55—who possess excellent bone quality and a morphological profile conducive to large-diameter bearings. This procedure is specifically indicated for a degenerative hip joint characterized by intractable pain and a progressively decreased range of motion that has failed conservative management. When joint-preserving procedures such as femoral osteotomy, periacetabular osteotomy, or vascularized bone grafting are deemed not ideal for an early stage of unambiguous coxarthrosis, hip resurfacing emerges as the premier reconstructive option. It is generally reserved for patients with robust bone stock, a high functional activity level, and an occupational or recreational need for a high degree of motion.

The general indications for hip resurfacing encompass a spectrum of degenerative pathologies. Primary osteoarthritis remains the most common indication, provided the cystic changes within the femoral head are minimal. Secondary osteoarthritis as a sequela of childhood diseases, including mild hip dysplasia or treated pediatric infection, can also be addressed, though these cases require meticulous preoperative templating. Post-traumatic arthritis, ankylosing spondylitis, and cautiously selected cases of rheumatoid arthritis are also viable indications. However, osteonecrosis of the femoral head presents a uniquely challenging problem. The presence of a necrotic lesion can severely compromise the cement interdigitation and fixation of the femoral component. In general, survivorship of hip resurfacing is demonstrably lower for avascular necrosis than for primary osteoarthritis, and its use in AVN is typically restricted to lesions involving less than 30% of the femoral head volume without massive structural collapse.

Contraindications to hip resurfacing are absolute and must be rigorously respected to prevent early catastrophic failure. Abnormal anatomy of the femoral head or neck, such as severe coxa vara or a shortened femoral neck, precludes the safe placement of the implant. A severe limb-length discrepancy that may require substantial lengthening during arthroplasty is a contraindication, as resurfacing offers minimal capacity for leg length adjustment compared to modular THA stems. Severe bone deficiency, encompassing systemic osteoporosis or localized osteopenia, dramatically increases the risk of femoral neck fracture and component subsidence. Large and cystic lesions of the femoral head that compromise more than one-third of the supportive bone stock contraindicate the procedure, as the remaining bone cannot adequately support the cemented mantle. Furthermore, severe acetabular dysplasia is a strict contraindication because the requisite supplemental screw fixation for the acetabular shell cannot be performed with standard resurfacing cups, which rely solely on equatorial press-fit.

Clinical Decision Matrix for Hip Resurfacing

Pre-Operative Planning, Templating, and Patient Positioning

Exhaustive preoperative planning is the cornerstone of a successful hip resurfacing arthroplasty. The margin for error is exponentially smaller than in traditional THA. Standard-sized conventional radiographs, including a low anteroposterior (AP) pelvis and a cross-table lateral of the affected hip, must be utilized. If digital films are employed, the magnification factor (typically 115% to 120%) must be precisely calibrated and taken into account to avoid catastrophic sizing errors. Acetabular templating is performed first to determine the optimal center of rotation and to ensure that the selected cup size fills the acetabular fossa completely while achieving an aggressive equatorial press-fit. An acetabular component should be selected that not only fits the native acetabulum but also accommodates a femoral component size appropriate for the patient's femoral neck diameter.

Femoral component alignment is arguably the most critical preoperative consideration. Templates are utilized to determine approximate component sizes, ensuring the femoral component is sized such that there is absolutely no over-reaming of the femoral neck, which would risk notching and subsequent fracture. Conversely, oversizing must be avoided to preserve acetabular bone stock and prevent anterior iliopsoas impingement. Varus positioning must be strictly avoided; a neutral or slight valgus alignment (typically 135 to 140 degrees) is strongly preferred. When optimum femoral template positioning has been achieved, the surgeon measures the distance from the tip of the greater trochanter to the pin insertion point on the lateral femoral cortex (often around 60 mm) using the ruler printed on the template. The relationship between the exit point of the guidewire on the template relative to the lesser trochanter must be meticulously noted and reproduced intraoperatively.

To surmount the intrinsic limitations of manual techniques and alignment guides, computer-assisted navigation systems have been developed and widely integrated into resurfacing workflows. A computer-assisted navigation system is an intraoperative image-guided localization system used to enable proper cutting of bone and exact spatial location of the implants. Although hip resurfacing arthroplasty has been used for decades with acceptable results without navigation, the persistent concern of femoral neck fracture has driven technological adoption. Intraoperative navigation can demonstrate real-time positioning of instrumentation by tracking arrays, thereby improving the accuracy of component positioning and effectively eliminating outliers in version and inclination.

However, navigation is not without its challenges. Specific bony surface landmarks may be difficult to digitize accurately through minimally invasive incisions, and registration procedures often represent the most time-consuming and accuracy-dependent segment of the operation. Irregular soft tissue distribution around the femoral neck can also affect navigation precision. When using an image-free hip navigation system, such as the Vectorvision system (BrainLAB, Munich, Germany) utilized by many advanced centers since 2005, the surgeon must carefully digitize the pelvic and femoral planes to establish the individual coordinate system. Despite these technical hurdles, surgical navigation is especially helpful during femoral procedures to avoid notching and ensure the targeted valgus alignment, thereby significantly reducing the potential risk of postoperative femoral neck fracture.

Step-by-Step Surgical Approach and Fixation Technique

Hip resurfacing is exceptionally technically demanding, and the surgical approach utilized must allow for adequate, circumferential exposure of both the acetabulum and the proximal femur without compromising postoperative abductor muscle function or the tenuous vascular supply to the femoral head. All of the standard exposures of the hip have been successfully utilized to perform surface replacement, though they each carry distinct advantages and liabilities. The anterior approach, originally advocated by Wagner, is generally not popular due to the extreme difficulty in mobilizing the proximal femur for cylindrical reaming without causing iatrogenic fracture. The anterolateral approach, described by Hardinge and modified by Learmonth, achieves femoral exposure via flexion, adduction, and external rotation. This approach preserves the posterior retinacular vessels, theoretically reducing the possibility of postoperative AVN. However, critics argue that the exposure is often inadequate, leading to a higher risk of component malpositioning, increased rates of heterotopic ossification, and compromised abductor function resulting in a prolonged Trendelenburg gait.

The trans-trochanteric approach, popularized by Amstutz, provides unparalleled, extensile exposure of the joint. However, it is rarely used in contemporary practice primarily because of the profound complications associated with trochanteric osteotomy, including non-union, wire breakage, trochanteric bursitis, and a significantly higher incidence of heterotopic ossification. Consequently, the posterior or posterolateral approach remains the standard and most popular approach used for total hip resurfacing globally. The patient is placed in the lateral decubitus position, rigidly secured with pelvic positioners. The hip is flexed to 45 degrees, and a straight incision is made centered on the posterior edge of the greater trochanter. The fascia lata is incised, the fibers of the gluteus maximus are split bluntly, and a Charnley retractor is deployed.

In the posterolateral approach, the tendon of insertion of the gluteus maximus is often partially released to allow for easy anterior displacement of the femur; underlying perforator vessels from the profunda femoris must be meticulously coagulated. The trochanteric bursa is divided and swept posteriorly, protecting the sciatic nerve. The short external rotators (piriformis, obturator internus, and gemelli) are tagged and released near their insertion. Crucially, unlike conventional THA, the joint capsule and synovial membrane around the femoral neck should be preserved as much as possible to reduce vascular damage. A capsulotomy is performed, and the femoral head is gently dislocated using flexion, adduction, and internal rotation, avoiding excessive torsion that could fracture the neck.

Once dislocated, the femoral neck is meticulously measured using specialized neck gauges to confirm preoperative templating. This sizing dictates the minimum acetabular size required. Usually, the acetabular preparation is performed first to ensure optimal visualization. The acetabulum is reamed to a bleeding subchondral bone bed, and the highly porous, uncemented acetabular shell is impacted via a press-fit mechanism, aiming for 40-45 degrees of inclination and 15-20 degrees of anteversion. If the femoral head is exceptionally large, resulting in inadequate space for acetabular preparation, the femoral head may be partially machined first. Following acetabular fixation, the femoral head is prepared using a sequenced cylindrical reamer and chamfer cutters centered over a precisely placed guide pin. The pin must reflect the templated valgus angle. Following preparation, the femoral head is thoroughly lavaged and dried. High-viscosity polymethylmethacrylate (PMMA) bone cement is applied, and the femoral component is impacted until fully seated, ensuring a uniform cement mantle and avoiding any entrapment of soft tissue.

Complications, Incidence Rates, and Salvage Management

Despite rigorous patient selection and meticulous surgical technique, hip resurfacing carries a unique complication profile that diverges significantly from traditional THA. The most devastating early complication is femoral neck fracture, which typically occurs within the first six months postoperatively. The incidence ranges from 1% to 3% in large registry data. Fractures are predominantly mechanical in origin, resulting from intraoperative neck notching, varus malalignment of the femoral component, or unrecognized osteonecrosis compromising the structural integrity of the supporting cancellous bone. When a femoral neck fracture occurs, salvage management necessitates immediate revision to a standard stemmed total hip arthroplasty. Fortunately, because the femoral canal was not violated during the index procedure, a primary THA stem can usually be utilized, though a large-diameter modular head must be matched to the retained resurfacing acetabular shell, provided the shell is well-fixed and optimally positioned.

A more insidious and highly publicized complication is the development of Adverse Local Tissue Reactions (ALTR), encompassing Aseptic Lymphocyte-Dominated Vasculitis-Associated Lesions (ALVAL), metallosis, and pseudotumor formation. These biological reactions are driven by the release of nanometer-sized cobalt and chromium wear particles from the MoM bearing surface. The incidence of symptomatic pseudotumors is highly variable but is significantly elevated in patients with edge-loading secondary to steep acetabular cup inclination (>50 degrees), small component sizes (femoral heads < 48mm, predominantly affecting female patients), and specific recalled implant designs (e.g., the ASR system). Patients typically present with unexplained groin pain, swelling, or a palpable mass. Diagnosis requires a combination of serial serum metal ion testing (Cobalt and Chromium levels > 7 parts per billion are concerning) and Metal Artifact Reduction Sequence (MARS) MRI to visualize cystic or solid periarticular masses.

Component loosening is another mode of failure. Femoral loosening is often secondary to thermal necrosis from cement curing, inadequate initial fixation, or progressive AVN of the underlying bone. Acetabular loosening is less common but can occur if initial press-fit stability is not achieved, particularly since resurfacing cups lack the option for supplemental screw fixation. Impingement between the femoral neck and the rim of the acetabular component can lead to early loosening or dislocation, underscoring the absolute necessity of preventing malposition of both components.

Complication Incidence and Salvage Protocols

Clinical & Radiographic Imaging Archive