Challenging the Lewinnek Safe Zone
Acetabular cup placement in the functional safe zone, as defined by the combined sagittal index, may be a better indicator of hip stability in total hip arthroplasty patients, according to a recently published study.
Emilie R. C. Williamson, MD, and Ajit J. Deshmukh, MD
Tezuka T, Heckmann ND, Bodner RJ, Dorr LD. Functional safe zone is superior to the Lewinnek safe zone for total hip arthroplasty: why the Lewinnek safe zone is not always predictive of stability. J Arthroplasty. 2019 Jan;34(1):3-8. doi: 10.1016/j.arth.2018.10.034. Epub 2018 Nov 2. PubMed PMID: 30454867.
Several studies have reported that placing the acetabular cup in the Lewinnek safe zone – a parameter of acetabular inclination and anteversion in the coronal plane on standing pelvic radiographs – does not ensure hip stability in total hip arthroplasty (THA) patients. The acetabular inclination (40° 10°) and anteversion (15° 10°) indicated in the Lewinnek safe zone is not representative of cup changes during functional activities such standing from a sitting position, which may be measured on lateral sitting and standing radiographs.
The change in cup position, rotating in the sagittal plane, is directly related to spinopelvic motion:
- The pelvis tilts anteriorly while standing, allowing the acetabulum to cover the femoral head.
- The pelvis tilts posteriorly while sitting, allowing the acetabulum to open in anteversion and permit clearance of the femur.
If there is spinopelvic imbalance, the resulting cup positions may not keep the cup within the Lewinnek safe zone.
In a prospective, cohort, observational radiographic study, Tezuka et al sought to determine if acetabular cups implanted within the Lewinnek safe zone are always concomitantly within the functional safe zone as defined by the combined sagittal index (CSI), which is equal to the sum of the anteinclincation (AI) and pelvic femoral angle (PFA).
The study included 320 hips (290 patients) with an acetabular cup within the Lewinnek safe zone. All patients underwent THA using computer navigation for placement of the cup. Combined sagittal index outlier values were used to identify hips at risk of dislocation, with standing CSI outliers greater than 243° and sitting CSI outliers less than 151°.
Using a Pearson’s correlation coefficient or odds ratio, the authors analyzed the relationship between coronal and sagittal safe zones in:
- Hips within the Lewinnek safe zone
- Hips within the functional safe zone (CSI)
- Hips within the Lewinnek safe zone and functional safe zone
- Hips within the Lewinnek safe zone but outside the functional safe zone (CSI outliers)
- Hips outside the Lewinnek safe zone but within the functional safe zone
- Hips outsize both the Lewinnek safe zone and the functional safe zone
They also analyzed preoperative measurements predictive of hips within the Lewinnek safe zone but outside the functional safe zone:
- Change in sacral slope (SS)
- Femoral motion as defined by change in preoperative femoral motion (PFA)
- Pelvic incidence (PI)
Increased PFA was the best predictor of hips being outside the functional safe zone. The second preoperative predictor for hips being outside the sitting functional safe zone was pelvic motion stiffness, or low change in SS. A third predictive factor was PI, which was less predictive than PFA or change in SS.
Increased PFA as measured on lateral pelvic radiographs was the best predictor of hips being outside the functional safe zone. This helps to explain why studies have not been able to identify the acetabulum as the primary cause of dislocation. The correlation of femur motion, not acetabular motion, with impingement is important to consider.
The second preoperative predictor for hips being outside the sitting functional safe zone was pelvic motion stiffness, or change in SS of less than 11°. A combination of increased PFA and pelvic stiffness increased the risk of dislocation: The pelvis cannot move out of the way of the femur, allowing for greater impingement to occur.
The third predictive factor was PI, as hips with low PI require more femoral flexion to sit.
Dual mobility articulation implants may be used in hips with all 3 – increased femoral flexion, stiff pelvic motion, and low PI – as there is no coronal safe zone that can be used to avoid the risk of impingement.
Not all hips within the Lewinnek safe zone were also within the functional safe zone. This suggests that preoperative planning with appropriate radiographs should aim for implanting components in the functional safe zone rather than the Lewinnek safe zone.
The study has some limitations. Tezuka et al did not correlate the discrepancy between coronal and sagittal cup position with dislocation, as this study did not investigate dislocations. Thus, the study did not validate the relationship between dislocation and CSI but suggests that mechanical complications may be related to sagittal motion of the pelvis and femur.
In view of this limitation, future studies are needed to further validate the relationship between the functional safe zone and hip instability/dislocation.
Emilie R. C. Williamson, MD is an orthopaedic surgery resident at NYU Langone Orthopaedic Hospital, New York, New York. Ajit J. Deshmukh, MD is an assistant professor of orthopaedic surgery, Division of Adult Reconstruction, Department of Orthopaedic Surgery, at NYU Langone Orthopaedic Hospital, New York, New York. He is also an orthopaedic surgeon at the VA New York Harbor Healthcare System, New York, New York.
The authors have no disclosures relevant to this article.