In this study, it was found that in patients receiving an MP prosthesis, PTS was positively correlated with postoperative ROM. With the increase of PTS, the activity level increased significantly. When the increase to PTS reached a certain angle, the impact on ROM and activity level plateaued. Increasing the level of PTS within an appropriate range was also shown to have no negative impact on joint stability.
Tibial intramedullary localization refers to placing the osteotomy guide mold in the tibial medullary cavity, the premise is that the long axis of the medullary cavity coincides with the anatomical axis of the tibia on the coronal and sagittal planes. Although intramedullary localization has the advantages of simple operation, easy assembly, and the positioning process was not susceptible to external interference, it would inevitably damage the medullary cavity structure, increase bleeding, and especially increase the risk of fat embolism. When the tibia is abnormally bent or the medullary cavity is offset, the positioning is not accurate, so the intramedullary localization is greatly restricted in clinical practice. Compared with intramedullary positioning, extramedullary localization has fewer complications. With some surface markers, it can meet the requirements of determining the force line of the lower limbs. It is simple and easy to implement, so it has become the preferred positioning method for most orthopedic surgeons [32].
Up to now, TKA depends to a certain extent on the operator's visual judgment and empirical operation to determine tibial osteotomy. Before tibial osteotomy, the positioning function of the tibial extramedullary locator is very important. The tibial extramedullary guide should be parallel to the long axis of the tibia no matter the frontal view or the side view. However, due to the physiological curvature of the tibia, despite strict extramedullary positioning, the placement of the prosthesis after tibial osteotomy still has a certain degree of deviation, so there is a significant difference between postoperative and preoperative PTS.
A good ROM is essential for normal tasks in daily life, for example, the knee joint of a normal patient needs to flex between 60–70°when walking and 90–120° when walking up and down stairs or sitting in the chair. As well as this, patients may require an increased ROM for religious activities and movements associated with Asian culture, such as cross-legged sitting and squatting [21.22]. Although a greater ROM is theoretically more beneficial for the patient in their daily life, excessive postoperative flexology will increase patellofemoral pressure, which may lead to anterior knee pain, excessive wear of the replacement joint, fracture of the patellar and other complications [23]. Therefore, surgeons generally consider performing an osteotomy to put the PTS in a suitable range for normal individuals, to restores normal knee kinematics as much as possible.
Some research reported that the angle of the PTS in a normal knee falls roughly in the range of 4–13 ° [24.25]. Matsuda [26] found that PTS in normal knee and arthritic knee joints fell in this range, however in a Chiu et al study assessed the PTS in Chinese patients with osteoarthritis was 2 to 3 °larger than this “normal” range [27]. Although a small number of studies suggested that increasing the PTS did little to improve the ROM and knee joint function after surgery [28.29], it was generally thought that a PTS of 10 °or lower improves the ROM and does not impact stability. In this study, when the PTS is maintained at 5–7°, there is sufficient range of motion for the posterior femoral condyle when the knee is flexing, resulting in a significantly higher range of motion in this group of patients than in patients with PTS less than 5°. As the PTS of the knee joint continues to ben, the raised posterior lip of the MP prosthesis pad will limit further movement of the posterior femoral cortex, which will in turn limit any improvements to postoperative ROM.
The size of PTS also relates to the amount of tibial plateau osteotomy. The tibial plateau is an important load bearing structure of the knee joint, the PTS in the normal range allowed the femoral condyle to roll and slide normally in the joint, which ensure good stability and flexion during the extension of the knee joint. When the anteroposterior diameter of the tibial platform is constant, increasing the PTS increases the amount of surface removed in the anterior tibial cortical osteotomy, that will expose more fragile bones in the front of the surgical site and lead to a loosening of the tibial prosthesis. There was also a research that if the PTS was too small, it may cause too much bone to be removed during the surgery, this can cause increased pressure on the bone which caused the tibial prosthesis to sink [30]. Performing the cut parallel to the surface of the tibial platform rather than perpendicular to the long axis of the tibial platform provided a 40% higher load bearing capacity, this increases the stiffness by 70% which is enough to support the stability of the tibia prosthesis. The size of the PTS also has a significant impact on the placement space of the tibial component. The osteotomy profile and osteotomy area under different PTS sizes are different, which may further affect the suspension or cause a defect of the prosthesis, limiting its service life.
Joint instability and aseptic loosening are the main reasons for TKA revision. Aseptic loosening is mainly caused by mechanical and biological factors. Interface fretting and stress splintering of the surrounding bone are some mechanical [31]. PTS plays an important factor in reducing the chance of these mechanical stresses. The final interface is one of the main sites of potential wear and interacts with the lateral tibia bone, increasing the size of the PTS increases the coverage of the implant interface. If the PTS is too low the size of the interface is decreased, this concentrates the stress of movement on a smaller area. This can lead to early wearing of the prosthesis, damage to the supporting bone, aseptic loosening of the prosthetic and cause eventual failure of the implant. Increasing the interface area between the bone and the implant can improve initial stability of the implant following TKA, the reduction in load bearing stress also reduces the incidence of aseptic loosening and increases the life span of the implant. Therefore, increasing PTS not only improves the postoperative ROM but may prove to be beneficial in improving postoperative outcomes. In this study, one patient from group C later had follow up surgery to replace the gasket due to recurrent joint pain. It was considered that the PTS was too big, this leads to buckling clearance being too loose. At the same time, the increased PTS influenced the tension of both sides of the collateral ligament, which caused the tibia and femur articular to loosen gradually. In light of this, it is recommended that increasing PTS to improve knee flexion should be done carefully and stay within the recommended range to avoid joint instability.
Under the same PCO condition, for every 6°increase in PTS, the maximum force on the quadriceps femoris and patellar tendon decreased by 34%. Adding PTS and PCO can enlarge the posterior position of the femoral component, the further back the contact position between the component parts is, the larger the quadriceps lever arm is, thus improving motion efficiency by reducing quadriceps muscle strength required and patella button contact stress, while also reducing extensor device problems [32.33]. Ostermeier [32] et al. found that in the case of a tibial posterior dip angle of 10°, less quadriceps force was required to apply the same stretching moment, especially when the knee flexed above 60 °. Kang [33] et al. also concluded through computer simulation that with the increase of PTS, the force of quadriceps femoris and patellar tendon would decrease under any condition of PCO and the force applied to the posterior cruciate ligament would also decrease with a decrease of PTS.
Current research is aimed at PS prostheses, when PS prostheses are involved in moderate bending of the knee, the femoral part of the prosthesis will slide forward and produce the contradictory movement. Only when the cam collides with the column will the ideal rollback motion be produced, this column-cam mechanism is relied on instead of PCL. Due to the possible joint instability during bending and the fact that the curvature of the design can lead to further soft tissue tension and instability, it is suggested thar an MP lip be and before and after the tibial gasket. This provides more stability during bending of the knee joint. During follow-up, we found that for most of the elderly, the priority outcome for the surgery was a reduction in pain and improved stability, whilst allowing for enough ROM to meet daily requirements. Therefore, we believe that for patients receiving an MP prosthesis, the PTS should be kept between 5–7°. Not only does this achieve an ideal activity level (108°), it avoids the pain and instability problems caused by the large PTS. As Ranawat reported that, a high degree of flexion (140–150°) whilst being stable and painless is the exception rather than the rule [23].
Although in this study, we tried our best to match the three groups of patients’ BMI index, PCO, joint line height, preoperative ROM and used patients who were operated on by the same surgical team, there were still some inevitable factors that may cause errors, which was one of the limitations of this study. In addition, due to the wear and degradation of articular cartilage before the surgery, measures of PTS based on the articular surface may not fully reflect the PTS of the knee joint, this may cause errors to the measurement technique.