What are the key considerations for a total hip replacement surgery, including preoperative evaluation, surgical approach, and postoperative rehabilitation?

Hip Biomechanics for Total Hip Replacement Surgery

Core Biomechanical Principles

Understanding hip biomechanics is essential to restore native anatomy and achieve a stable hip during primary arthroplasty, as proper prosthetic placement directly determines both radiological and clinical success. 1, 2

Critical Biomechanical Parameters to Restore

• Center of rotation must be accurately reconstructed to replicate the native hip joint mechanics and prevent abnormal loading patterns 2
• Femoral offset (the perpendicular distance from the center of the femoral head to the long axis of the femur) must be restored to maintain proper abductor muscle tension and hip stability 2
• Acetabular offset (the distance from the center of rotation to the pelvis) affects joint reaction forces and must be optimized to reduce component wear 2
• Acetabular inclination should be positioned within safe zones (typically 30-50 degrees) to minimize dislocation risk and edge loading 2
• Acetabular anteversion (typically 15-25 degrees) is crucial for preventing impingement and instability 2
• Leg length discrepancy must be minimized (ideally <5mm) to prevent gait abnormalities, patient dissatisfaction, and increased litigation risk 2

Consequences of Biomechanical Malpositioning

• Dislocation is the catastrophic complication resulting from improper component positioning, with high patient morbidity 1
• Reoperation rates increase significantly when any biomechanical element is malpositioned 2
• Accelerated wear occurs with suboptimal acetabular positioning, particularly excessive inclination or anteversion 2
• Impingement between the femoral neck and acetabular rim develops when safe zones are violated 1

Preoperative Evaluation for Biomechanical Planning

High-Risk Patient Identification

• Comprehensive preoperative evaluation identifies patients at increased risk for instability, allowing surgical plan modification before entering the operating room 1
• Patient factors including neuromuscular disorders, cognitive impairment, prior hip surgery, and abductor muscle deficiency increase instability risk 1
• Anatomical variations such as acetabular dysplasia, femoral version abnormalities, or spinopelvic pathology require templating adjustments 1

Radiographic Templating

• Preoperative templating on anteroposterior pelvis and lateral hip radiographs allows prediction of implant size and positioning to restore native biomechanics 2
• Measurement of native anatomy including femoral offset, leg length, and acetabular anatomy guides intraoperative reconstruction 2

Intraoperative Biomechanical Assessment

Stability Testing Protocol

• Routine intraoperative stability assessments must be employed to correct any surgical misjudgments before closure 1
• Range of motion testing in flexion, extension, internal rotation, and external rotation identifies impingement or instability 1
• Component position verification using fluoroscopy or navigation ensures acetabular and femoral components are within safe zones 1

Implant Selection Based on Biomechanics

• Knowledge of implant options is imperative for achieving stability, including dual-mobility constructs for high-risk patients, constrained liners for recurrent instability, and modular femoral components for offset restoration 1
• Bearing surface selection (metal-on-polyethylene, ceramic-on-ceramic, ceramic-on-polyethylene) affects long-term implant survival and should be chosen based on patient age, activity level, and biomechanical demands 3

Postoperative Biomechanical Considerations

Activity Modification Based on Biomechanics

• High-intensity activities (tennis, squash, snowboarding) do not increase revision rates but may correlate with increased hip pain in some patients 4
• Low-impact activities (swimming, aquafit, cross-country skiing) are less likely to cause hip pain and maintain implant longevity 4
• No restriction on activity intensity is necessary from a biomechanical standpoint, as higher-intensity activities do not lead to decreased activity duration or increased revision rates 4

Radiographic Follow-up

• Postoperative radiographs should be analyzed for center of rotation, femoral offset, acetabular offset, acetabular inclination, acetabular anteversion, and leg length discrepancy to verify biomechanical restoration 2
• Serial radiographs detect component migration, osteolysis, or wear that may indicate biomechanical failure 2

Common Biomechanical Pitfalls

• Excessive acetabular inclination (>50 degrees) increases edge loading and accelerates polyethylene wear 2
• Insufficient femoral offset restoration reduces abductor muscle tension, causing Trendelenburg gait and instability 2
• Leg length inequality >10mm significantly impacts patient satisfaction and functional outcomes 2
• Combined anteversion errors (femoral + acetabular anteversion outside 25-50 degrees) dramatically increase dislocation risk 1