Lewinnek Safe Zone
Dislocations after total hip-replacement arthroplasties
Lewinnek GE · J Bone Joint Surg Am · 1978
In this landmark retrospective study of 300 primary total hip arthroplasties, Lewinnek and colleagues established the classic acetabular component 'safe zone'—defined as 40° ± 10° of radiographic inclination and 15° ± 10° of radiographic anteversion—which was associated with a significant reduction in post-operative prosthetic dislocation.
Citation & links
Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am. 1978 Mar;60(2):217-20.
Key findings
- 1.A total of 9 post-operative dislocations (3.0%) were observed among the 300 total hip replacements evaluated.
- 2.Acetabular components positioned within 30° to 50° of radiographic inclination (40° ± 10°) and 5° to 25° of radiographic anteversion (15° ± 10°) defined the designated 'safe zone'.
- 3.Cups positioned outside this safe zone demonstrated an approximate fourfold higher dislocation rate (6.1%) compared to cups placed inside the zone (1.5%).
- 4.Anterior hip dislocations were strongly correlated with excessive component anteversion and vertical inclination.
- 5.Posterior hip dislocations occurred frequently even when the socket appeared within normal parameters, highlighting the compounding role of femoral version, soft-tissue laxity, and surgical approach.
Background
Instability and dislocation represented one of the most frequent and debilitating complications following total hip arthroplasty (THA) in the early Charnley era. The exact technical and orientation parameters required during acetabular socket implantation to provide optimal prosthetic stability remained poorly defined. The authors investigated the relationship between three-dimensional radiographic orientation of the acetabular cup and the incidence of post-operative hip dislocation.
Methods
A retrospective review of 300 consecutive total hip arthroplasties performed via a posterior or transtrochanteric approach at a single institution. Post-operative anteroposterior and true lateral radiographs were analyzed using mathematical trigonometric calculations to compute true radiographic inclination and anteversion relative to the anterior pelvic plane. Component orientations in hips that sustained post-operative dislocations were statistically compared to those that remained stable.
Clinical implications
The Lewinnek 'safe zone' became the global surgical benchmark and target for acetabular socket positioning during primary THA for over four decades. While it led to widespread adoption of orientation targets to limit prosthetic impingement and edge-loading, contemporary arthroplasty literature (such as Abdel et al., 2016) has shown that up to 80% of modern dislocating hips fall within these boundaries. Today, it is recognized as a population-level guideline rather than an absolute guarantee of stability, shifting modern practice toward patient-specific functional component positioning and spinopelvic mobility considerations.
Critical appraisal
Strengths include the pioneering mathematical and radiographic methodology that first quantified three-dimensional acetabular component orientation relative to pelvic anatomy. Major limitations include its retrospective study design, a very small sample size of dislocations (n = 9), static 2D radiographic assessment in the supine position without accounting for standing/sitting spinopelvic kinematics, lack of femoral version assessment (combined anteversion), and reliance on historical implant designs and large femoral heads.
Teaching note
Every orthopaedic candidate must instantly recall Lewinnek's values: 40° ± 10° inclination and 15° ± 10° anteversion. In viva examinations, always quote these figures as historical radiographic targets, but score higher marks by critically emphasizing that contemporary literature demonstrates that most unstable hips dislocate within the Lewinnek safe zone due to spinopelvic mechanics, patient posture, combined anteversion, and soft-tissue balance.
Exam-tested points
- ›Acetabular safe zone definition: Radiographic inclination of 40° ± 10° (range 30°–50°).
- ›Acetabular safe zone definition: Radiographic anteversion of 15° ± 10° (range 5°–25°).
- ›Dislocation risk outside the safe zone was reported as approximately 4 times higher (6.1% vs 1.5%) in the original cohort.
- ›Original safe zone calculations were referenced to the Anterior Pelvic Plane (APP) in the supine position.
- ›Posterior dislocations can still occur within the safe zone, frequently mediated by femoral retroversion, posterior capsular deficiency, or dynamic pelvic tilt.
- ›Modern spinopelvic biomechanics supersedes static safe zones: a stiff lumbar spine requires dynamic, functional component adjustment to prevent posterior impingement in flexion or anterior impingement in extension.
- ›Abdel et al. (CORR 2016) demonstrated that ~58% of contemporary dislocating THAs were fully within the Lewinnek safe zone, proving it is not universally protective.
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