Comparing Different Systems for High Tibial Osteotomies
Comparing Different Systems for High Tibial Osteotomies
The knee is one of the most heavily loaded joints of the human body during daily activities. For the HTO surgery, the medial opening is an extremely unstable condition for the proximal tibia, and the fixation device is used to stabilize the opening and enhance bone union. This study used the one- and two-leg plates to evaluate the effects of locking screw and plate leg on the construct stress and wedge micromotion. For the one-leg design, the screw, plate, and bone stresses of the TomoFix construct were respectively 42.2%, 31.9%, and 35.6% less than those of the T construct (Figure 5). For the two-leg design, the aforementioned stresses of the π plate can be reduced by 20.3%, 11.9%, and 26.5% as compared with the T+I construct. This indicated that the use of locking screws can significantly reduce the mechanical demands of the implants and surrounding bone.
The contact behavior at the plate-screw interfaces can be used to account for the effects of the locking screws (Figure 7). The head of the nonlocking screw can freely rotate within the plate hole but the threads of both the screw head and plate hole were tightly locked. Due to the intimate contact, the screw and plate stresses of the locking systems (i.e. TomoFix and π) could be more uniformly distributed and reduced (Figure 5). Upon loading, the nonlocking screw potentially rotates and makes line or even point contact with the plate hole (Figure 7b). This led to the highly concentrated stresses at the screw-plate interfaces of the nonlocking systems (i.e. T and T+I). The biomechanical study of Seide et al. demonstrated that the stable fixation at the screw-plate interfaces can distribute the bony loads more uniformly and avert stress concentration at some local sites. This can be used to explain the reduction of bone stress of the TomoFix and π systems (Figure 5).
(Enlarge Image)
Figure 7.
The schematic diagrams to show the difference in contact area between the locking and nonlocking screw-plate constructs: (a) Locking screw (b) Nonlocking screw.
(Enlarge Image)
Figure 8.
The legs of the π system develop an equivalent force-couple to resist the knee loads.
Two plate-leg factors enhanced the biomechanical performance of the HTO construct: more screws to support the knee loads and the wider supporting base below the opening. The two-leg system provides more screws to stabilize the medial opening, thus significantly decreasing the plate and bone stresses and suppressing the wedge micromotion (Figures 5 and 6). For the T+I and π systems, there are two nearly parallel legs to transmit the knee loads through the opening (Figure 1). The current authors hypothesized that the plate serves as a fulcrum to transmit the knee loads from the proximal to the distal bones. With respect to the sagittal plane, the nonuniform distribution of the knee loads potentially induced a counterclockwise moment to the tibial plateau (Figures 3b and 8). The original loads and induced moment were balanced by the plate and remaining cortex at the wedge tip (i.e. edge cc). The two plate legs can reconstruct a wider supporting base to behave as an effective force-couple mechanism. The anterior leg is mainly subject to the tension loads and the majority of the compression loads transmits through the posterior leg. The moment arm spanned by the legs can reduce the moment-induced stresses of bone and implants and wedge-tip micromotion (Figure 6).
For the two-leg systems, the load ratio of the posterior to anterior legs can provide biomechanical information of the load-transferring mechanism. If the tops of both T and I plates were linked, the value of the load ratio could be decreased from 4.7 to 3.9. This indicated that the anterior leg of the π plate could share the loads of the posterior leg to reduce the mechanical failure at that location. Meanwhile, the results of the load ratio demonstrated the effective distribution of the enormous loads from the knee joint to the distal tibia.
The height changes in the edges aa, bb, and cc were regarded as the indices of the construct stability. Except for the π plate, the micromotions of the other systems consistently exceeded the reported maximum value (> 100 μm) of the allowable movement for the bone union. However, some studies have shown that the highly rigid fixation may cause osteoporosis due to the stress-shielding effect. Moreover, adequate micromotion of fracture interfaces can enhance the callus formation. Historically, the trade-off between rigid fixation and interfacial micromotion is still unknown.
For the one-leg systems, the wedge micromotions were significantly higher that the counterparts and were contributed to the bone-screw loosening and construct instability. At the wedge tip (edge aa) of the one-leg systems, Figure 6 showed the occurrence of bone separation that results from the leverage effect and negatively affects the callus formation. In the literature, some researchers proposed the anteromedial side as the optimal position of the one-leg plate to stabilize an osteotomized tibia. On the other hand, follow-up and cadaveric studies of the opponents found that the anteromedial placement induces posterior tilt of the tibial plateau and construct instability. Comparatively, the two-leg plate provides a greater area to cover the circumferential sides of the medial opening (Figure 1). This can deter the placement-induced effects of the one-leg plate on the construct responses and ensure more stable support to the opening.
There are some limitations inherent in this study. The use of both physiological and surgical loads may be overestimated for the early stage of the HTO healing. After surgery, however, the treated leg was often partially loaded by using a crutch or a walker. If shorter bed-resting time was desired, this study can provide the worst-case comparison of the different HTO plate systems in the situation of the full body-weight. From the biomechanical viewpoint, both bone variations and wedge sizes might alter the stabilizing effect of those HTO plates. However, these were not included in this study and the experimental validation should be performed.
Discussion
The knee is one of the most heavily loaded joints of the human body during daily activities. For the HTO surgery, the medial opening is an extremely unstable condition for the proximal tibia, and the fixation device is used to stabilize the opening and enhance bone union. This study used the one- and two-leg plates to evaluate the effects of locking screw and plate leg on the construct stress and wedge micromotion. For the one-leg design, the screw, plate, and bone stresses of the TomoFix construct were respectively 42.2%, 31.9%, and 35.6% less than those of the T construct (Figure 5). For the two-leg design, the aforementioned stresses of the π plate can be reduced by 20.3%, 11.9%, and 26.5% as compared with the T+I construct. This indicated that the use of locking screws can significantly reduce the mechanical demands of the implants and surrounding bone.
The contact behavior at the plate-screw interfaces can be used to account for the effects of the locking screws (Figure 7). The head of the nonlocking screw can freely rotate within the plate hole but the threads of both the screw head and plate hole were tightly locked. Due to the intimate contact, the screw and plate stresses of the locking systems (i.e. TomoFix and π) could be more uniformly distributed and reduced (Figure 5). Upon loading, the nonlocking screw potentially rotates and makes line or even point contact with the plate hole (Figure 7b). This led to the highly concentrated stresses at the screw-plate interfaces of the nonlocking systems (i.e. T and T+I). The biomechanical study of Seide et al. demonstrated that the stable fixation at the screw-plate interfaces can distribute the bony loads more uniformly and avert stress concentration at some local sites. This can be used to explain the reduction of bone stress of the TomoFix and π systems (Figure 5).
(Enlarge Image)
Figure 7.
The schematic diagrams to show the difference in contact area between the locking and nonlocking screw-plate constructs: (a) Locking screw (b) Nonlocking screw.
(Enlarge Image)
Figure 8.
The legs of the π system develop an equivalent force-couple to resist the knee loads.
Two plate-leg factors enhanced the biomechanical performance of the HTO construct: more screws to support the knee loads and the wider supporting base below the opening. The two-leg system provides more screws to stabilize the medial opening, thus significantly decreasing the plate and bone stresses and suppressing the wedge micromotion (Figures 5 and 6). For the T+I and π systems, there are two nearly parallel legs to transmit the knee loads through the opening (Figure 1). The current authors hypothesized that the plate serves as a fulcrum to transmit the knee loads from the proximal to the distal bones. With respect to the sagittal plane, the nonuniform distribution of the knee loads potentially induced a counterclockwise moment to the tibial plateau (Figures 3b and 8). The original loads and induced moment were balanced by the plate and remaining cortex at the wedge tip (i.e. edge cc). The two plate legs can reconstruct a wider supporting base to behave as an effective force-couple mechanism. The anterior leg is mainly subject to the tension loads and the majority of the compression loads transmits through the posterior leg. The moment arm spanned by the legs can reduce the moment-induced stresses of bone and implants and wedge-tip micromotion (Figure 6).
For the two-leg systems, the load ratio of the posterior to anterior legs can provide biomechanical information of the load-transferring mechanism. If the tops of both T and I plates were linked, the value of the load ratio could be decreased from 4.7 to 3.9. This indicated that the anterior leg of the π plate could share the loads of the posterior leg to reduce the mechanical failure at that location. Meanwhile, the results of the load ratio demonstrated the effective distribution of the enormous loads from the knee joint to the distal tibia.
The height changes in the edges aa, bb, and cc were regarded as the indices of the construct stability. Except for the π plate, the micromotions of the other systems consistently exceeded the reported maximum value (> 100 μm) of the allowable movement for the bone union. However, some studies have shown that the highly rigid fixation may cause osteoporosis due to the stress-shielding effect. Moreover, adequate micromotion of fracture interfaces can enhance the callus formation. Historically, the trade-off between rigid fixation and interfacial micromotion is still unknown.
For the one-leg systems, the wedge micromotions were significantly higher that the counterparts and were contributed to the bone-screw loosening and construct instability. At the wedge tip (edge aa) of the one-leg systems, Figure 6 showed the occurrence of bone separation that results from the leverage effect and negatively affects the callus formation. In the literature, some researchers proposed the anteromedial side as the optimal position of the one-leg plate to stabilize an osteotomized tibia. On the other hand, follow-up and cadaveric studies of the opponents found that the anteromedial placement induces posterior tilt of the tibial plateau and construct instability. Comparatively, the two-leg plate provides a greater area to cover the circumferential sides of the medial opening (Figure 1). This can deter the placement-induced effects of the one-leg plate on the construct responses and ensure more stable support to the opening.
There are some limitations inherent in this study. The use of both physiological and surgical loads may be overestimated for the early stage of the HTO healing. After surgery, however, the treated leg was often partially loaded by using a crutch or a walker. If shorter bed-resting time was desired, this study can provide the worst-case comparison of the different HTO plate systems in the situation of the full body-weight. From the biomechanical viewpoint, both bone variations and wedge sizes might alter the stabilizing effect of those HTO plates. However, these were not included in this study and the experimental validation should be performed.
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