Abstract
Introduction
Excellent long-term survival rates associated with the absence of stem subsidence have been achieved with total hip arthroplasty (THA) using femoral components cemented line-to-line (“French Paradox”). Recently, short stems have been introduced in order to preserve diaphyseal bone and to accommodate to minimal invasive THA and a variety of clinical situations. The aim of the current study was to quantify the rotational and tilting stability of a Kerboull stem of varying length after line-to-line cementation using a validated in-vitro model.
Materials & methods
The femoral component made of M30NW stainless steel was derived from the original Kerboull stem. It had a double taper, a highly polished surface, and a quadrangular cross-section. Four stem lengths were designed from the original length with a distal reduction of 6, 12, 17 and 22%, whereas the proximal body geometry of the implant remained unaffected. For each stem length, five specimens were implanted into a non-canal synthetic femoral model. The femoral preparation was performed in order to obtain rotational and tilting stability of the stem prior to the line-to-line cementation. Spatial micro-motions of the specimens were investigated using a validated rotational measuring set-up. In addition, in a second separate step, the specimens were exposed to a ventro-dorsal moment to mimic varus-valgus moment. Statistical analysis was performed using ANOVA with Fisher PLSD.
Results
The maximum torque transfer from the stem within the cement mantle to the composite femur occurred at the level of the lesser trochanter, whereas the lowest torque transfer was observed at the tip of the stem. The relative movement at the tip was significantly greater for the original length when compared to 6 and 12% length reduction (p = 0.036 and 0.033, respectively). The 12% reduction resulted in a significant lower mean overall movement when compared to the original length (p = 0.044). The tilting behavior according to the stem lengths indicated that proximal bending value was significantly increased for 17% reduction when compared to 6% and 12% reduction (p = 0.035 and 0.032, respectively). Bending of the tip of the stem was in the same direction as the shoulder, indicating a backlash from the tip. At the tip, relative bending was increased when compared to the previous length up to 12% reduction and then decreased. However, the difference was not significant (p <0.05).
Discussion & conclusion
The stem lengths evaluated in the current study showed similar results to previously reported cemented stems of different designs, indicating a close fixation to composite bone with small relative movement. Both 6 and 12% shortened versions showed significant reduced relative movement at the tip when compared to the original length, suggesting a limited role for the tip in terms of rotational stability. Regarding the medio-lateral torque, the stems always reacted with a backlash and did not tilt like a rigid body. Although not significant, the distal bending tended to increase with reduced length. These findings led us to develop a Kerboull stem with 12% distal reduction that is currently under clinical trial.
