Before a clinical study in living patients is conducted, biomechanical studies need to be conducted to confirm the feasibility of the surgical technique and to test the effectiveness of the system in laboratory conditions. The majority of the parameters reviewed in this biomechanical study showed no significant differences between study groups. However, the load to failure was significantly higher for the new implant (r-pfFS) and cFS than for the standard press-fit femoral stem (pfFS). Cementless femoral stems with a transfixing rod offer significantly higher stability than do standard cementless stems, without significant differences between the stems in transverse displacement. r-pfFSs may be valuable, as they limit the compression and torsional forces and promote bone healing.
Cemented stems have better primary stability, with values that were 2 and 1.5 times higher than those for pfFSs and r-fFSs, respectively, and the difference between the cFS and pfFS groups was significant . Studying the “interface” is fundamental to understanding how femoral stems remain bonded to the femoral shaft. When materials with different properties (bone, cement, implant) come in contact, an interface is created. The load of the implant is therefore distributed to the surrounding bone along the bone-implant interface. Material properties are defined by stiffness, shape, and surface architecture. The load transitions between materials created by the interface need to bear large amounts of stress, and their ability to near this stress is defined by the stiffness ratio of each material . On the cemented stem, three interfaces are present: the cement-implant interface, inner cement mantle, and cement-bone interface. These interfaces permits the distribution of load on the different elements. Moreover, excellent primary stability can be possible by the cohesive action of the PMMA, which acts as grout. Bone cement penetrates the micro-irregular grooves of the reamed bones and is responsible for the shear forces at the interface. However, cement acts as a foreign body: polypropylene debris can migrate into the cancellous bone, enhancing the pro-inflammatory response and thus improving the risk of aseptic loosening .
In press-fit femoral stems, the initial fixation is obtained by press-fitting an oversized femoral stem in the femoral shaft to create primary stability with only frictional forces. Moreover, the stiffness of the cementless implant being higher than that of the bone leads to overloading of the implant and thus stress shielding of the bone. The results obtained in this study are in agreement with those in the literature [13, 14]. Work necessary for subsidence was lower for the pfFS than for the cFS and r-pfFS, although the differences were not significant. Indeed, although frictional forces can bear initial loading, load to failure and thus the work necessary for subsidence was significantly lower for the pfFS. This difference may be related to the lack of secondary fixation, such as fixation with cement, which acts as grout for the cemented stem and the rod for the r-pfFS.
The addition of a transfixing rod in neutralization in the neck of a cementless press-fit femoral stem allows the resistance to subsidence to be significantly higher than that of the standard cementless stem and the stability after cyclic loading to be similar to that of the cemented femoral stem. Bucks et al. compared the resistance of subsidence in a standard press-fit femoral stem and an interlocking femoral stem, and the peak load to failure was always significantly higher for the interlocking stem than for the standard press-fit femoral stem: 2.337 ± 782 N and 1.405 ± 712 N, respectively . The resistance to subsidence was greater with a transfixing neutral rod on the press-fit cementless femoral stem than with the standard femoral stem. Clinically, the results are consistent with those of the study by Mitchell et al., in which different types of cementless stems were analysed . Although the systems are not the same but are biomechanically similar, the lateral bolt femoral stem was associated with less subsidence in the postoperative period than was the standard stem. The rod strengthens the system by limiting the compression and torsional forces during axial loading, significantly increasing the solidity of the system without the need for additional complex surgical procedures. These results are encouraging; the r-pfFS system is similar to that of the cFS in terms of primary stability, with a similar magnitude of work necessary for subsidence. Moreover, the rod could be easily and quickly placed in the lateral cortex in the single hole under direct guidance by the prosthetic femoral neck, without inducing fracture or altering the stem’s anteversion angle, emphasizing the feasibility of the surgical technique. The procedure appears to be less complicated than that for other hybrid implants, such as interlocking femoral stems. However, the simple adjunction of the rod could lead to less stability and an increased risk of rod migration than do the other systems [13, 14]. Furthermore, there are no data on how well the rod stays in place in the long term. Indeed, whether the micromotion of the stem during gait can move or break the rod, increasing its potential for migration, has yet to be determined. It is nevertheless essential to perform the drilling procedure with care to prevent damage to the sciatic nerve, which is close to the surgical site .
Proper positioning of the femoral stem is a critical aspect during surgery as well as during biomechanical assays and is strongly related to the surgeon’s skills at the moment of positioning and impaction of the stem. In this study, all stems were implanted by a single experienced, board-certified surgeon. All the stems were well positioned with varus-valgus and craniolateral angles in concordance with those reported in the literature, and there were no significant differences between groups. This parameter needs to be assessed before any assays are performed. Indeed, if only subjective eye evaluations are performed after implantation, slight differences in angulation can affect the final result. It has been shown that varus angle of the femoral stem greater than or equal to 5° leads to an increased risk of fracture intraoperatively because of the medial position of the proximal part of the femoral stem, which overpressures a common site of fractures, the craniomedial part of the proximal femur .
The neutral position of the femoral stem permits an ideal distribution of the strain. The hole drilled in the trochanteric fossa maximizes the chance of adequate angulation of the stem in the femoral shaft. After head and neck ostectomy, the proximal part of the femur has a typical “8” shape formed by the intact greater trochanter and the distal part of the neck. The centre of the “8” shape corresponds to the trochanteric fossa and is in the same plane as the anatomic axis of the femur for optimal placement of the stem. If a hole is reamed on the lower circle of the “8” shape, the hole will naturally exhibit varus angulation, following the direction of the femoral neck, which can lead to incorrect positioning of the stem. The enlargement of the trochanteric fossa therefore provides good angulation of the stems. For press-fit prostheses, the position of the stem can have low precision; indeed, placing the stem with a hammer induces high variability in the stem position at the moment of impaction.
All the fractures in the groups were long, oblique fractures and were similar to fractures generally observed to lead to natural complications . However, the location of the fractures differed between groups. In the pfFS group, the fractures were on the medial aspect of the femur, with its origin on the craniomedial part of the proximal femur. This location represents the most common site of fractures, and it is generally due to the varization of the femoral stem [17, 19]. The absence of a uniform force distribution might cause fractures in this area. During failure tests, the stem might bend in relation to the femoral shaft, increasing its varus angle and reaching a critical position until failure. Unfortunately, no digital images were available to measure the displacement during the failure tests. In the cFS and r-pfFS groups, the fractures were located on the cranial aspect of the femur. The good neutral position confirmed by the post-implantation radiographs suggests that the forces were equally distributed between the two groups and emphasizes the advantages of the neutral rod in r-pfFSs over pfFSs.
Cyclic assays have been developed with the technical capabilities of a servo-hydraulic press. The aim of the test was not to imitate the immediate postoperative normal gait of the dog after surgery but rather to pre-stress the femur before the failure test. Immediately postoperatively, dogs walk for approximately 1500 steps . For the assays presented in this study, only 90 cycles were performed. Considering the relatively low number of cycles, the load was set to be 75% of the dog’s living body weight, and the trot was mimed to increase the stress on the femur . No significant difference was observed between all groups. Moreover, the p-value corresponding to the difference between the cFS and r-pfFS groups was highly non-significant, indicating a similar magnitude of transverse displacement. These results validate the reliability of the new implant and demonstrate that the new implant yields the same strain and displacement as the other groups. The results are concordant with those in previous studies, with values of 0.70 ± 1.21 mm and 0.35 ± 0.41 mm for the standard press-fit femoral stem and with the interlocking nail femoral stem, respectively .
However, this study has several limitations. The standard size of the femoral stems used in this study can be a limitation. However, it appears that the canals filled by stems are poor indicators for identifying good-sized stems and carry poor clinical relevance . The relatively low number of femurs studied can influence our statistical results. Moreover, the low number of cycles achieved in the assays may not precisely characterize the immediate postoperative period in living dogs. As a biomechanical study is always a simplification of what truly occurs in nature, the femurs were not subject to physiologic forces encountered during normal canine gait. Indeed, the actions of the gluteal and adductor muscles and the resulting rotational and shear forces were not taken into account during the assays. Moreover, the axis of the femur during the compression tests did not correspond to the physiological axis of the femur during weight-bearing in the animal’s daily life.