Abstract
Purpose
The positon of short stem is affected by the native anatomy of femoral neck and also by fixation mechanism dependent on design. As a consequence, it has been speculated that restoration of hip geometry might be limited in total hip arthroplasty (THA) using short stem. Therefore, the present study assessed the predictability of restoration of hip geometry using two different CCD-angled short stem engaging the lateral cortex
Materials and Methods
The 60 patients included 15 females and 45 males. The average age was 48.0 years with average BMI 24.2. Biomechanical parameters of hip geometry were analysed on postoperative calibrated radiographs in 30 consecutive primary unilateral THAs using short stem (Metha®, B. Braun Aesculap, Tuttlingen, Germany) with 120° CCD angle (group I) and 30 match controlled cases with 135° CCD angle (group II) and compared to those of the contralateral hip without deformity. The matching process was done before collecting the radiographic measurements by two blinded observer and was for sex, age ± 5 years, and BMI ± 7 units in that order.
Results
Head length was short in 40%, 67%, medium in 37%, 23% and large in 23%, 10% of the patients in each group respectively with no significant difference in between group (p=0.11). The discrepancies of horizontal hip center of rotation (△HHCR) and the vertical hip center of rotation (△VHCR) compared to the contralateral side was similar in both groups (p=0.95, p= 0.11, respectively), which enabled to make a direct comparison of the femoral reconstruction.
Compared to the contralateral side, discrepancies of limb length (△LLD) showed a borderline significant difference between two groups (avr.+0.7mm, +2.5mm respectively, p=0.04) with higher values for group of 135° CCD angle (more than 5mm of LLD in 27%). However, in group of 120° CCD angle, the discrepancies of horizontal femoral offset (△HFO) and abductor lever arm (△AbLA) (avr. +5.9 mm, +4.9mm respectively) revealed significantly increased compared with balanced value of group 135° CCD angle (+0.9mm, p <0.0001, +1.3mm, p=0.02, respectively) and about half of patients in group of 120° CCD angle revealed outside the 5mm difference target in either horizontal femoral offset (53% of patient) and abductor lever arm (50% of patient)
Conclusion
With decreasing CCD-angle of short stem, restoration of limb length appears more predictable but, horizontal femoral offset and abductor lever arm increased with outside of a beneficial range. This tendency should be taken into consideration when choice the design of this kind of neck-preserving short stem as well as exact implantation technique.
