The comparison of the two different types of prosthetic stems showed, that the stem with three different femoral neck-shaft angles and a constant femoral neck length could reconstruct the individual anatomical parameters significantly better for the horizontal and vertical offset as well as for the femoral neck-shaft angle and tendentially better for the leg length.
The better reconstruction of the anatomical parameters by the CoreHip stem is in our view founded on two characteristics of this stem system. First, by using three different CCD angle variants of the stem, that have the same medial calcar fit, the corresponding anatomical offset can be reconstructed without affecting the leg length (Fig. 2). Second, with the CoreHip system, the prosthetic neck length does not increase with increasing stem thickness, as is the case with many other stem systems on the market (including the Exception stem studied here). This corresponds to the pattern found in nature, since in nature there is no linear relationship between femoral neck length and the femoral canal width [13,14,15,16]. Moreover, especially in women with progressive osteoporosis, the femoral canal can become wider over the course of a lifetime, but the other anatomical parameters such as offset, femoral neck length and leg length remain the same . Especially in the latter condition, the use of a stem-system with increasing neck lengths with stem sizes would result in a significant increase in the offset.
An increase in the offset can have clinical consequences. It leads to tightness of the iliotibial ligament, which in turn can lead to irritation of the gluteal muscles at the greater trochanter and result in bursitis trochanterica . An increase in the horizontal femoral offset after hip prosthesis implantation can also lead to a change in the leg axis with a resulting change in the forces on the knee joint and even to unilateral osteoarthritis of the knee . On the other hand, a reduction of the offset reduces the tension of the gluteal muscles, which can lead to a Trendelenburg sign or gait . In addition, the distance of the trochanter major to the os ileum and that of the trochanter minor to the os ischium is reduced, and this can lead to bony impingement or even result in dislocation of the hip joint . Furthermore, a reduced offset increases the wear of polyethylene in the cup [25,26,27]. Therefore, it seems that the correct reconstruction of the offset has clear clinical advantages.
In addition to the offset, the leg length plays a crucial role in the reconstruction of the anatomical parameters. A changed leg length leads to pelvic tilt, which, if not compensated for by insoles or shoe adjustment, can cause problems in the lumbar spine and irritation of the N. ischiadicus [28,29,30]. In addition, differences in leg length may cause gait insecurity, dislocation and premature loosening of the prostheses after hip replacement [31,32,33]. Patient dissatisfaction with leg length discrepancies often leads to legal disputes [24,25,26,27,28,29,30,31,32,33,34,35,36,37]. Although the length of the prosthesis neck also has an influence on the leg length, the reconstruction of the leg length in this study was not significantly different between the two different stem systems. However, with the CoreHip, the leg length tended to be better reconstructed (11% more often) within a 5 mm difference to the non-operated opposite side. The lack of significance with regard to leg length, despite significant differences in the reconstruction of the vertical femoral offset, may in our opinion be due to the fact that the surgeons selected a larger or smaller stem to achieve the same leg length by placing the stem higher or lower in the femoral canal and could thus influence the leg length directly. The femoral offset was only slightly influenced by this. In addition, the use of modular prosthetic heads helps to achieve the correct insertion length.
From the explanations given, it seems evident that the reconstruction of the individual anatomy of the hip with the endoprosthesis is important and seems to be realized significantly better with a stem with three different femoral neck-shaft angles and a constant femoral neck length in different sizes. However, further studies should be aimed at determining whether the better reconstruction of the individual anatomical parameters also leads to different clinical outcomes.
The study has some limitations. First of all, this is a study of total endoprostheses. As such, the placement of the prosthetic cup also has an effect on leg length. Since only femoral vertical and horizontal offset was measured in this study and the same cup was used for both prosthetic stems, we believe that there is negligible influence of prosthetic cup placement on the significant differences detected in this study.
Moreover, the selected standard stem is not representative of all standard stems on the market because each stem has some specific features in neck-shaft angle and the increase in neck length with increasing stem size. However, it seems that an increase of neck-length with stem size generally does not reflect the natural anatomical relationship between the meta- and diaphyseal canal width and the femoral neck length [13,14,15,16].