gms | German Medical Science

Objectives: Lewinnek's Safe-Zone gives recommendations for cup placement in total hip arthroplasty that are still widely accepted although they do address the socket only while ignoring the stem completely. The criteria are not clearly defined and the ranges for cup orientation are considerably large. This study introduces the new combined safe-zone (cSafe-Zone) which gives clear directions for the optimal combined orientation of both components in order to reduce the risk for prosthetic impingement and maximizing the intended range of movement (iROM). This cSafe-Zone also yields the basis for the combined version concept and stem-first surgical technique.

Methods: Preventing prosthetic impingement was chosen as the criterium to determine the orientation of the components while achieving the predefined iROM. A 3D-computer model of a total hip prosthesis was established that does systematically modify all design parameters that affect the range of motion. The tests were carried out for straight stems, anatomic stems and short stems. The iROM was defined as the entity of basic hip movements like flexion/extension, internal/external rotation and ab/adduction that should be reached by the patients and that are commonly accepted as physiologic hip movements. The analysis did reveal all parameter-dependent combinations of component orientations that are satisfying this predefined iROM and as such are defining the new combined safe-zone.

Results: The new combined safe-zone has a dynamic location and has a polygonal outer boundary. It is smaller than Lewinnek's safe-zone. Its size and location within the cup inclination/anteversion diagram depends on the antetorsion of the stem. It can be demonstrated that a low-anteverted stem should be combined with a high-anteverted cup and vice versa, i.e. cup anteversion and stem antetorsion are linearly but inversely correlated. This is true for a straight stem as well as for anatomic and for short stems. The size of the cSafe-Zone is best when the socket is radiographically anteverted between 20° and 25°. The neck/shaft-angle (CCD-angle) and the anatomic design do have a high impact on the preferred antetorsion of the stem. A straight 130°-CCD-stem is best implanted in 20°+/-4degree of antetorsion while an anatomic 127°-CCD-stem should be implanted in only 8°+/-4degree of antetorsion. Increasing the head/neck ratio gives room for lowering the cup inclination.

Conclusion: The new combined safe-zone (cSafe-zone) gives well-defined recommendations for cup and stem placement taking into account the dynamic interrelationship between cup and stem. In extending Lewinnek's recommendations it defines how both the cup and the stem should be oriented relative to each other and how the component's orientations are optimized in order to achieve the highest safety against prosthetic impingement while reaching the highest range of movement which is especially important for high-performance materials and in young and active patients.