Forgotten but relevant: The origins of the “safe zone” in total hip arthroplasty

Paper in Focus: 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. PMID: 641088. BACKGROUND The concept of a “safe zone” for acetabular cup positioning, first described by George E. Lewinnek and colleagues in 1978, remains one of the most influential ideas in total hip arthroplasty (THA). Based on radiographic evaluation of 300 primary THAs, Lewinnek et al. proposed that acetabular inclination of 40° ± 10° and anteversion of 15° ± 10° potentially minimize the risk of postoperative dislocation. Despite advances in navigation, robotics, and three-dimensional imaging, these parameters continue to serve as essential reference values. This short commentary revisits the historical context, methodological simplicity, and enduring relevance of this classic study – acknowledging both its limitations and its timeless contribution to the science of component orientation.[1] HISTORICAL CONTEXT In the 1970s, as Charnley’s low-friction arthroplasty gained worldwide adoption, instability became a frequent and frustrating complication. Radiographic analysis tools were rudimentary, and most surgeons relied on visual cues to orient the acetabular cup. At the University of Southern California, Lewinnek and colleagues undertook a systematic analysis of 300 consecutive THAs to quantify the geometric parameters associated with dislocation. Their simple but disciplined methodology based on standard radiographs culminated in a discovery that would shape decades of surgical teaching. THE CORE FINDINGS The study identified acetabular component position as the most significant modifiable factor linked to dislocation. From this, Lewinnek et al. proposed a “safe zone”: Inclination: 40° ± 10° Anteversion: 15° ± 10°. Hips placed within these limits demonstrated a markedly reduced incidence of dislocation. This defined, for the first time, a measurable target for cup positioning and introduced the vocabulary – “safe zone,” “anteversion,” “inclination” – that remains in use today [Figure 1].Figure 1: Illustrative diagram showing Lewinnek’s safe zone: Inclination 40° ± 10°, anteversion 15° ± 10°, and the contemporary functional safe zone conceptINFLUENCE ON PRACTICE The Lewinnek safe zone is embedded in operative manuals, residency teaching, and implant design. For almost 40 years, this numeric window has guided cup orientation in both cemented and uncemented THA. It also provides a benchmark for emerging technologies – navigated, robotic, and artificial intelligence-assisted systems – which still report accuracy as the percentage of implants “within the Lewinnek zone.” Its conceptual simplicity unified generations of surgeons in the pursuit of reproducible biomechanics and stability.[2,3] CRITIQUE AND EVOLUTION Contemporary analysis, aided by computed tomography and motion capture, revealed that the original study’s radiographic method carried measurement error and neglected patient-specific variables such as spinopelvic mobility, femoral version, and sagittal pelvic tilt. Studies by Callanan et al. (2011) and Domb et al. (2014) demonstrated that up to 40% of stable hips lay outside the classic zone, while some within it still dislocated [Table 1]. It was also observed that there are significant racial variations and cultural expectations that may preempt universal application of safe zones.[4] These observations gave rise to the improvized concept of the functional safe zone that was personalized as per patient’s anatomy and surgeons’ own assessment of the patients’ need and cultural preferences. “Functional safe zone” – a dynamic, individualized range reflecting the patient’s posture and spinal mechanics rather than fixed angles.[5] Yet, even this evolution traces its intellectual roots to Lewinnek’s original insight: stability is geometric and measurable [Figure 2].[6]Figure 2: Dynamic variation of acetabular orientation across postures and functional plane determination using modern navigation and preoperative templating tools like three-dimensional modeler by Smith and NephewTable 1: Comparison between the classical Lewinnek safe zone and the modern functional safe zone conceptsThe functional safe zone integrates [Table 1]: Spine–pelvis–hip continuum Dynamic motion patterns Patient-specific planning. Instead of a static radiographic target, it is a kinematic envelope of stability tailored to each patient.[7] The concept of functionality takes into the account the spinopelvic mobility and the pelvic tilt. The modern software like Hip Modeler also enables surgeons to take into account the impingement profile and patient anticipated activities [Table 2].Table 2: Patient-specific spinopelvic considerations and recommended adjustments to acetabular cup orientationCONTEMPORARY RELEVANCE Despite five decades of progress, the safe-zone principle remains foundational. Whether a cup is placed by freehand technique or through navigation and robotics, orientation is still quantified using Lewinnek’s coordinate system. The 1978 study’s enduring value lies not merely in its numbers but in its philosophy – that precision in positioning is the essence of prevention. In today’s era of digital planning, Lewinnek’s paper is a reminder that accuracy begins with awareness, not just with algorithms. Functional implications and adjustments to the standard Lewinnek zone however can add to the patient-specific personalization [Table 2]. Key message The “safe zone” concept introduced by Lewinnek in 1978 was the first systematic attempt to define measurable boundaries for acetabular orientation. Although modern imaging and functional alignment have refined it, every cup placed today still owes its coordinates to this pioneering study. Key finding: “The dislocation rate for cup orientation with anteversion of 15° ± 10° and lateral opening of 40° ± 10° was 1.5%, while outside this “safe” range, the dislocation rate was 6.1%.” Footnote: This series “Forgotten but Relevant” is an attempt to bring together landmark papers in hip and knee arthroplasty with a contemporary relevance. We welcome comments and letters for this section to initiate an active dialogue VB and MP.