Hip Instability

Brian P. Chalmers

Robert T. Trousdale

Key Concepts

Careful patient evaluation is important to identify clinical and physical risk factors that place a patient at risk for instability. These include lumbar spine pathology, abductor mechanism dysfunction, underlying neuromuscular disorders, and cognitive disorders.
Careful radiographic evaluation of the lumbar spine, implants, component fixation, component position, hip offset, and hip length is paramount.
Address instability in revision total hip arthroplasty in a step-wise fashion:
- Fixed bearing.
  - Address component malposition, soft tissue and bony impingement.
  - Restore soft tissue tension with hip length and offset.
  - Utilizing a large-diameter fixed-bearing femoral head increases jump distance and range of motion free of impingement, thereby increasing hip stability.
- Dual mobility construct.
  - Increases the effective femoral head diameter and the arc of motion before prosthetic impingement and may be preferred in high-risk patients.
- Constrained liners.
  - Reserved for severe abductor and soft tissue insufficiency and/or failure of the above-mentioned approaches.

Sterile Instruments and Implants

Basic hip retractors and instrumentation.
Acetabular explant system.
Acetabular component position tester.
Implant-specific femoral heads and modular polyethylene liners, including lateralized, lipped, and face-changing liners.
Revision uncemented acetabular components of choice (with instrumentation), including large-diameter femoral head articulations.
Revision uncemented femoral component of choice.
Dual mobility construct of choice (with instrumentation).
Constrained liner of choice (with instrumentation).

Surgical Approaches

Positioning—lateral decubitus position.
Both the posterior and anterolateral approaches may be used as they may be made extensile, allowing for wide exposure, if needed. as it is reproducible and extensile.
The direct anterior approach, in expert hands, is useful for a minority of revision hip arthroplasties, but extensile exposure is more difficult.
The senior author reserves the anterolateral approach in those with severe abductor damage seen at the time of exposure.

Preoperative Planning

Analysis of Patient Clinical Risk Factors

Analyze patient demographics—elderly patients and women are more prone to dislocation (Figure 39.1).
Consider comorbid conditions:
- Neurologic conditions, e.g., Parkinson disease, Charcot arthropathy, cerebral palsy, post poliomyelitis.
- Lumbar spine fusion/stiffness, especially extending to the pelvis.
Recognize the indication for primary total hip arthroplasty—posttraumatic (e.g., prior femoral neck fracture) and avascular necrosis have higher risk for postoperative dislocation.

Analysis of Physical Examination Risk Factors

Abductor strength/abductor lurch.
Limb length, both actual and perceived, discrepancy.
Stiffness of the lumbar spine.

Radiographic Evaluation—Compare Preoperative and Current Radiographs

Polyethylene wear/osteolysis.
Component fixation.
Acetabular component position (Figure 39.1).
- Ideal acetabular inclination = 40° (Figure 39.2).
- Ideal acetabular anteversion = 15° to 20° (Figure 39.2).
  - Should be analyzed on a true/direct lateral radiograph of the hip.
Cup position may vary based on lumbar disease.
Hip length and offset (Figure 39.3).

Figure 39.1 ▪ A patient with a posterior-superior total hip dislocation with an overly abducted acetabular component.

Figure 39.2 ▪ Radiographs showing important measurements to assess component position and restoration of hip offset and length. The ideal acetabular inclination is 40° (A). The ideal acetabular anteversion is 15° to 20° (B).

Prior Operative Reports

Prior Surgical Approach

Guides surgical planning and better understanding of instability mechanism, along with knowledge of the direction of recurrent dislocations.
- For example, if a primary total hip arthroplasty was done via a posterior approach but is dislocating anteriorly, close evaluation of acetabular and femoral anteversion should be completed.

Type of Articulation

Metal-on-metal articulations or concern for taper corrosion should prompt evaluation of metal ion levels and evaluation of the abductor mechanism (by physical examination and magnetic resonance imaging with neutral artifact reduction sequences).

Figure 39.3 ▪ Restoration of both hip offset (arrow, A) and hip length (arrow, B) compared with preoperative radiographs and the contralateral hip is also important to assess.

Implant Records

Be prepared with modular femoral heads and polyethylene liners that match implants that may be retained at operation.
Knowledge of whether there is a modular lateralized or face-changing polyethylene liner, dual mobility construct, or constrained liner option with the implant system.
Obtain implant-specific extraction devices as needed.

Surgical Preparation

Be Prepared With Surgical Multiple Options

Obtain the modular femoral heads and polyethylene liners of current implants if plan to obtain the original components.
Template a revision acetabular component and a revision femoral component and have them available before surgery if revision is indicated (e.g., loosening, component malposition,).
Have available dual mobility construct and/or constrained liner of choice if additional stability is needed.

Bone, Implant, and Soft Tissue Techniques