Rationale

Despite improved surgical techniques and implant designs, dislocation is still a leading cause for revision total hip surgery, inducing as many as one in five total hip arthroplasty (THA) revisions.¹,² This is a major concern for the patient and hip surgeon alike. Total hip arthroplasty patients experiencing dislocation require significant healthcare resources to evaluate and manage this complication.³,⁴ Ensuring that evidence‐based techniques are employed for THA reconstruction will translate into appropriate use of these limited resources.

Clinical comment

Dislocation through implant malposition is a dynamic problem. Many studies have demonstrated that the classic static target numbers as referenced to anatomical landmarks for implant positioning are insufficient to ensure stability, and functional studies of implant positioning are required.⁵–¹³ There is still no consensus towards the ideal target implant position, but data confirms the importance of incorporating pelvic functional tilt with combined femoral and acetabular version and femur first preparation to maximize range of motion (ROM) and minimize the risk of implant impingement and dislocation .¹⁴–¹⁹ Patients with combined pelvic‐spine deformities are at high risk for bone on bone impingement despite avoiding implant impingement and require additional workup.

Understanding an algorithm with evidence‐based investigations for common causes of impingement and pain in THA ensures that correctable problems are addressed.⁵,¹⁰,¹¹,²⁰,²¹

Available literature and quality of evidence

The quality of literature addressing appropriate investigations for functional implant positioning is highly variable with the vast majority being Grade II‐IV evidence, mainly case series or consecutive cohort studies. Dorr et al. analyzed the influence of pelvic mobility to cup position through conventional radiography and recommended standing and sitting anteroposterior (AP) and lateral pelvic radiographs for patients identified as high risk.⁸ Lazennec and Sariali used the EOS® radiographic technique which takes simultaneous capture of two full body orthogonal AP and lateral images and provides two‐ and three‐dimensional models using its Stereos® software.⁷,²² This allows superior functional analysis of hip‐spine deformities and implant position.²³–²⁵

Findings

Kanawade et al. confirmed the normal range of pelvic motion from sitting to standing is 20–35° with increase in anteinclination upon sitting being 25° inclination and 14° anteversion. Their studies of pelvic tilt confirmed that 20% of patients present with a stiff (≤20°) or hypermobile (≥35°) hip‐pelvic motion making them at risk for impingement and dislocation, respectively.⁸,²⁶ The particular preoperative mobility of the pelvis predicts the necessary modifications in strategy of implant position to minimize this risk. The fact that nearly 80% of THA are implanted near their functional position makes THA tolerant to minor errors in positioning and explains the excellent long‐term results that it has enjoyed.²⁷

Dropout dislocation occurs upon sitting in patients with 1) hypermobility – posterior pelvic tilt upon sitting (as their functional inclination can be near 80–90°) or 2) hips that are shortened upon reconstruction. Intraoperatively, stability against dropout is tested by pushing the hip to its maximal flexion to the chest and observing that it does not dislocate. These patients (mainly females) need the cup at 35–40° inclination and 25° anteversion so that combined anteversion (CAV) is about 40–45° as they are at risk for posterior dislocation. Stiff pelvis (mainly males) with little posterior tilt and low functional anteinclination needs a cup position around 40–45° inclination as they bend forward more (trunk flexion) from sitting to standing and 15° anteversion so that CAV is about 25–30° to minimize risk for anterior dislocation in extension upon standing.⁷,⁸,²⁸,²⁹ Additional measures such as trochanteric transfer may be required to minimize bone on bone impingement.¹²,¹³

The EOS® system was confirmed to achieve less radiation with accurate and reliable information for measuring spine acetabular, femoral and pelvic variables; it especially improves measurement of femoral offset and anteversion which standard 2D radiographs under‐value from external rotation contracture.²²,²³,³⁰

Resolution of clinical scenario

• Level II–IV evidence shows that 20% of patients present with pathologic pelvic‐spine mobility at risk for dislocation.
  
• Level III evidence confirms utility of preoperative functional radiographs or EOS imaging to work up at‐risk patients.
  
• Level II evidence confirms surgical strategies and modification of implant position are required for at‐risk patients.

Rationale

The range and type of pelvic tilt influences functional cup position.⁶–⁸,¹⁰ Thus, it is important to understand which surgical techniques allow for the most accurate functional implant positioning.

Clinical comment

Dislocation is the most alarming complication after THA, but pain, accelerated wear or loosening are also related to impingement and are probably clinically underestimated. In cementless THA, femoral anatomy dictates postoperative implant position, with about 20% of femurs in retroversion ≤0° (cam deformity), or ≥20° anteversion (dysplasia) and puts the patient at risk for impingement dislocation.²⁰,³¹,³² This was the basis for femur first preparation and the concept of CAV.

At present it is still difficult to quantify the implications of implant malposition on gait and stance,³³,³⁴ but greater knowledge in this field has opened new research opportunities.

Available literature and quality of the evidence

To analyze anatomy, surgeon perception, impingement and dislocation Dorr et al. studied a series of cohorts of over 200 patients with navigation and postop CT‐scans.²⁰,³¹,³⁵,³⁶ With cementless femoral implants femur‐first was used and cup anteversion was adjusted towards a combined anteversion of 25–45° while incorporating navigated pelvic tilt to try and achieve impingement free range‐of‐motion between stem and cup (level III and IV studies).³¹,³⁵ As discrepancies in definitions of target acetabular positioning in literature was found, laboratory models were developed to try and obtain consensus for reporting implant position, especially acetabular anteversion (level IV).¹⁶,³⁷ Zhu and Maratt et al. in a consecutive series of patients analyzed the influence of pelvic tilt on cup functional position.¹⁴,³⁸

Renkawitz et al. in an RCT compared navigated femur‐first technique with conventional THA to assess clinical functional outcomes (level II).³⁹ Weber in a prospective study of 135 patients evaluated 6 current definitions of combined anteversion for impingement free range of motion (level II).⁴⁰

Findings

Total hip arthroplasty using the combined anteversion technique with navigation found no dislocations and achieved the targeted combined anteversion in 96% of cases. Dorr et al. confirmed that even experienced hip arthroplasty surgeons with mechanical guides had outliers over 5° in 31% of cases for acetabular inclination, 39% for anteversion and 46% for the femur version.⁸,²⁰,³¹,³⁵–³⁷ Similar results were found from other studies of postoperative implant position with outliers of up to 50%.⁴¹–⁴³ Implant positioning depending on human experience alone is imprecise and inaccurate.³⁶,³⁹,⁴⁴–⁵²

Weber et al. concluded that standard rules of combined anteversion improved prosthetic range of motion in 90% of cases but failed to prevent combined osseous and prosthetic impingement in up to 40% of cases because of anatomic variants which occur in individuals.³³

Related posts:

The Direct Anterior Approach Management of Glenoid Bone Loss Distal Femur Fractures Adolescent and Adult Spinal Deformity: Operative Management Periprosthetic Fractures: Knee Lisfranc Injuries

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