Introduction / Definition

Hip resurfacing arthroplasty (HRA) is an alternative to conventional stemmed total hip arthroplasty (THA) designed to preserve proximal femoral bone stock and recreate native joint biomechanics. Rather than resecting the femoral head and neck, the femoral head is sculpted and capped with a hemispherical prosthetic shell, articulating against an uncemented monobloc metal acetabular cup.

Modern resurfacing relies on second-generation metal-on-metal (MoM) bearings fabricated from high-carbon, cast cobalt-chromium-molybdenum alloys. Earlier iterations pioneered by Charnley, Freeman, and Wagner using thin polyethylene acetabular liners failed catastrophically due to excessive volumetric polyethylene wear and subsequent osteolysis. Re-popularised in the late 1990s by McMinn (Birmingham Hip Resurfacing, BHR), modern HRA demonstrated excellent survivorship in select patient demographics, though international uptake has sharply declined due to concerns regarding adverse reactions to metal debris (ARMD) and implant recalls (e.g., ASR, DePuy).

Epidemiology & Demographics

Hip resurfacing accounts for less than 1% of primary hip arthroplasties recorded in contemporary national registries, such as the UK National Joint Registry (NJR) and the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR).

Registry evidence consistently demonstrates that implant survivorship is intimately linked to patient sex, femoral component size, and surgical volume:

• Ideal candidate profile: Active, young males (<60–65 years) with primary osteoarthritis, a femoral head diameter ≥48–50 mm, and normal proximal femoral anatomy.
• Poor prognostic profiles: Female sex, small component sizes (<46–48 mm), older age (>65 years), and underlying hip dysplasia. Registries note significantly higher 10- and 15-year revision rates in females, driven by smaller component diameters, edge-loading, and higher biological susceptibility to metal hypersensitivity.

Relevant Anatomy & Biomechanics

Femoral Head and Neck Vascularity

The primary blood supply to the femoral head traverses via the medial femoral circumflex artery (MFCA) through its deep branch and lateral retinacular (subcapital) vessels. During surgical exposure (notably via the posterior approach), aggressive dissection, extensive neck skeletonisation, or extensive circumferential capsulotomy can jeopardize lateral retinacular vessels, potentially predisposing to osteonecrosis under the femoral component.

Biomechanical Advantages

1. Proximal Femoral Bone Stock Preservation: Preserves the femoral neck and canal, facilitating future conversion to standard primary THA stems rather than revision constructs.
2. Large Bearing Diameter: Matches anatomical head size, resulting in a large jump distance and near-physiological range of motion without impingement, conferring an exceptionally low risk of prosthetic dislocation.
3. Physiological Stress Transfer: Loading occurs through the proximal femoral cortex and trabeculae in a compressive manner, reducing distal stress shielding typically observed with uncemented THA femoral stems.
4. Proprioception: Preservation of the femoral head core and joint mechanics fosters superior proprioception and functional gait velocity compared to traditional THA.

Classification & Indications

Patient Selection Spectrum

Strict adherence to selection criteria is paramount to clinical success:

• Ideal Indications:

  • Primary osteoarthritis in young, high-demand male patients.
  • Femoral head diameter ≥48 mm.
  • T-score > -1.0 (normal bone mineral density).

• Relative Contraindications:

  • Female sex of childbearing age (transplacental transfer of cobalt and chromium ions).
  • Femoral head cysts >1 cm affecting >30–50% of the head volume.
  • Moderate-to-severe hip dysplasia (Crowe II–IV; acetabular uncoverage).
  • Minor leg length discrepancies (>1–1.5 cm) which cannot be corrected by resurfacing.

• Absolute Contraindications:

  • Impaired renal function (inability to excrete cobalt and chromium ions leading to systemic toxicity).
  • Known systemic metal allergy (cobalt, chromium, molybdenum, nickel).
  • Severe osteopenia or osteoporosis (T-score < -1.5 to -2.0).
  • Extensive avascular necrosis (Ficat and Arlet stage III/IV involving >33–50% of head volume).
  • Significant proximal femoral deformity (Perthes disease, slipped capital femoral epiphysis, extreme retroversion).

Clinical Presentation & Examination

Candidates present identically to standard end-stage degenerative hip disease patients:

• History: Progressive, activity-related anterior groin pain, radiating into the buttock or anterior thigh/knee; morning stiffness; impaired ability to participate in sports, heavy manual labour, or activities of daily living.
• Physical Examination:
  • Antalgic or Trendelenburg gait pattern.
  • Restricted internal rotation in 90° flexion, along with reproducible groin discomfort on passive hip rotation.
  • Fixed flexion deformities or adduction contractures.
  • True vs. apparent leg length discrepancy assessment.
  • Thorough baseline neurovascular examination.

Investigations

Plain Radiographs

• Pelvis AP (standing) & Lateral (cross-table / shoot-through lateral or false-profile): Assesses joint space narrowing, Tonnis grade of osteoarthritis, presence and dimensions of subchondral cysts in the femoral head, femoral neck-shaft angle, and acetabular morphology (e.g., retroversion, lateral centre-edge angle of Wiberg).
• Contralateral hip AP: Aiding baseline sizing of the femoral head and acetabulum.

Dual-Energy X-Ray Absorptiometry (DEXA)

Indicated routinely in borderline demographics (males >55 years, borderlines with risk factors) to confirm normal bone mineral density (T-score > -1.0) before confirming operative plan.

Cross-Sectional Imaging (CT / MRI)

• CT: Utilised if marked subchondral cyst formation or dysplastic acetabular morphology is suspected, quantifying remaining load-bearing bone stock.
• MARS-MRI (Metal Artefact Reduction Sequence): The investigation of choice in painful post-operative HRA to detect soft tissue masses (pseudotumours), joint effusion, tendon avulsions (abductor mechanism), and capsule rupture.

Serum Metal Ions

• Baseline monitoring is not typically mandated pre-operatively. Post-operatively, whole blood or serum cobalt and chromium ion levels evaluate bearing performance. In the UK, the Medicines and Healthcare products Regulatory Agency (MHRA) threshold level of 7 parts per billion (ppb) flags potential adverse local tissue reactions, requiring close multidisciplinary follow-up.

Management

Non-Operative Management

Equivalent to established guidelines for end-stage hip osteoarthritis:

• Activity modification, low-impact exercise, and physiotherapy.
• Weight reduction counselling.
• Oral analgesia (paracetamol, non-steroidal anti-inflammatory drugs) according to NICE guidelines.
• Intra-articular image-guided corticosteroid or hyaluronic acid injections (with operative postponement for at least 3 months to mitigate infection risk).

Operative Technique

Surgical Approaches

• Posterior Approach (Moore/Southern): Most common approach. Requires meticulous preservation of the deep branch of the MFCA. Extensive capsulotomies should be executed judiciously. Soft tissue repairs of the capsule and conjoint short external rotators (piriformis, obturator internus, and gemelli) must be anatomical.
• Modified Anterolateral Approach: Avoids disruption of posterior circumflex retinacular vessels but provides more constrained visualization of the acetabulum, risking superior gluteal nerve branch traction and abductor weakness.

Step-by-Step Procedure

1. Exposure: Extensive mobilization is required to safely dislocate and externalize the femoral head anteriorly without creating excessive torsional or bending moments on the neck.
2. Femoral Head Preparation: A centrally placed guide wire is passed down the central axis of the femoral neck. A dedicated alignment jig is calibrated to ensure the component is placed in slight valgus (5°–10°) relative to native anatomy.
3. Avoiding Notching: Critical step; superior femoral neck notching must be avoided at all costs, as it creates a stress riser precipitating early fracture.
4. Femoral Reaming & Shaping: Cylindrical, planar, and chamfer cutters sculpt the head. Multiple drill holes or venting holes are made to decompress the marrow and assist cement interdigitation.
5. Acetabular Preparation: Unreamed acetabular preparation preserves circumferential subchondral bone. The hemispherical uncemented monobloc cup relies on line-to-line or minimal press-fit fixation. Optimal orientation requires an inclination of 40°–45° and anteversion of 15°–20° to prevent edge-loading.
6. Femoral Implantation: Low-viscosity acrylic bone cement is introduced onto the inside of the femoral shell or carefully applied to the reamed bone mantle, seating the component fully to avoid cement extruding down the femoral neck.

Complications

Femoral Neck Fracture

• Incidence: 1%–4% of cases, typically within the first 6–12 months post-operatively.
• Etiology: Superior femoral neck notching during reaming, placement in varus orientation (elevating tensile forces on the superior neck), avascular necrosis, or underlying unrecognized osteopenia.
• Treatment: Revision to a conventional primary stemmed total hip arthroplasty (acetabular component can often be retained if compatible and undamaged).

Adverse Reaction to Metal Debris (ARMD) / Pseudotumour

• Includes aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL), extensive metallosis, and sterile cystic/solid peri-articular inflammatory pseudotumours.
• Risk factors: Acetabular component malorientation (inclination >50–55° or excessive anteversion/retroversion resulting in micro-separation and stripe wear), smaller implant size (<48 mm), and female sex.
• Presentation: Pain, sensation of instability, clicking, audible squeaking, groin swelling, or compressive neuropathies (femoral or sciatic nerves).
• Treatment: Revision to ceramic-on-polyethylene or ceramic-on-ceramic THA, necessitating complete pseudotumour debridement and exchange of both bearing surfaces.

Avascular Necrosis (AVN) of the Resurfaced Head

• Arises from disruption of lateral retinacular vessels, heat-induced thermal necrosis from cement polymerization, or pressurisation-induced microvascular compromise.
• Presentation: Early to mid-term groin pain with mechanical subsidence or progressive radiolucencies under the femoral dome.

Aseptic Loosening

• Femoral: Primarily secondary to stress shielding, osteonecrosis, or deficient initial cement interdigitation.
• Acetabular: Loss of biological ingrowth into the porous-coated shell; often related to excessive early edge-loading.

Exam Pearls & Evidence

• The McMinn / Amstutz Alignment Principle: The femoral component must always be aligned in neutral or slight valgus (5°–10°) relative to the femoral neck axis. Varus alignment increases tensile loads across the superior cortex, dramatically escalating femoral neck fracture risks.
• Edge-loading & Contact Stress: Increased inclination (>50°) sharply reduces the functional clearance and fluid-film lubrication regime, shifting MoM lubrication from hydrodynamic/mixed lubrication into boundary lubrication, multiplying volumetric wear 10- to 100-fold.
• National Joint Registry (NJR) Data: The NJR confirms that modern BHR in young, active males with head sizes ≥50 mm demonstrates long-term survivorship (15 years) comparable or superior to stemmed THA (approaching 95% survival). Conversely, females with small heads (<46 mm) show revision rates approaching 20% at 10 years.
• Failure Modes: Unlike stemmed THA (where aseptic loosening, dislocation, and periprosthetic fractures dominate late follow-ups), early failure in HRA is dominated by periprosthetic femoral neck fracture, while mid-to-late failure is driven by ARMD and aseptic loosening.
• MHRA Surveillance Protocol: Symptomatic MoM hip patients or those with whole blood metal ions ≥7 ppb (cobalt or chromium) warrant MARS-MRI imaging and serial annual clinical and laboratory surveillance.