When is the knee ready for the prosthesis?
The symptoms of degenerative joint diseases are very variable. Advanced arthrosis on X-ray in almost asymptomatic patients is not uncommon. This contrasts with invalidating pain that takes the patient out of everyday life and confines him to bed.
The initial treatment of gonarthrosis in most cases involves conservative approaches. Physical therapy in combination with antiphlogistic measures are often successful. The complaints show a good response to intra-articular injections and the patient is happy about conservative measures, especially after initial confrontation with the diagnosis. However, if the complaints increase despite this treatment, the question of surgical options arises.
Knee arthroscopy is a sensible, minimally invasive treatment option for incipient arthrosis. Partial resections of degeneratively torn menisci or cartilage smoothing can certainly have a positive effect. However, if the pathological changes are advanced, joint replacement should be discussed. In particular, joint deformities with changes in the leg axis in the direction of valgus or varus should give reason to consider knee prosthesis as a therapeutic option, and evaluation by an orthopaedic surgeon is useful in these cases.
Development and new surgical technique
Knee prosthetics is on the rise with increasing numbers every year. There is an immense variety of different knee prostheses in line with the increasing demand worldwide. The basis of some of these models is the recreation of the physiological movement of a healthy knee. This pivoting around the medial part of the joint during flexion and extension movements is intended to give the patient as natural a feeling as possible of the operated knee.
Increasingly, the implantation of knee prostheses is performed after individual, bone-based planning using computer or magnetic resonance imaging. In many cases, the patient-specific cutting blocks prove to be a useful and time-saving technique. In the case of pathological varus or valgus leg axes, however, problems can arise because the ligamentous situation of the knee joint is not recorded in the planning. The resulting instability can be partially compensated with a soft tissue release. In the case of higher degree deformities, the position of the femoral component can be adjusted in a controlled manner in order to obtain a balanced knee prosthesis. It makes sense to adjust the leg axis and/or femoral rotation before the femoral bone cuts are made in order to prevent proximalization of the joint line.
One such technique is illustrated below. The correction of the positioning of the femoral component can be reliably performed before the femoral bone resection.
Leg axis and soft tissue balancing
Since the middle of the 19th century, attempts have been made to surgically repair arthroses. Themistokles Gluck marked the beginning of joint replacement in the knee area at the end of the 19th century. Since then, there has been a continuous development and optimization of implants and surgical techniques, which have led to the current state of prosthetics. While many innovations have had to be discarded, some principles have endured. These include the ligamentous balancing of the knee prosthesis and the adequate alignment of the leg axis.
Various studies have shown that a neutral leg axis is one of the important prerequisites for the functioning and long service life of knee prostheses. However, the neutral and physiological leg axis is an individual value that varies within certain limits. Healthy individuals with pre-existing hypovalgic or even slightly varicose alignments do not necessarily benefit from a knee prosthesis implanted in a standard valgus position.
Ligament balancing aims to achieve balanced mediolateral stability of the implanted knee prosthesis in flexion and extension. Preoperatively existing instabilities can be addressed in a first step by means of soft tissue release. Often, symmetrical ligament tension can be achieved while maintaining a neutral leg axis. In the case of pronounced deformities in the direction of valgus or varus, release is not always sufficient for complete balancing. In these cases, a slight adaptation of the femoral component position is useful. However, this correction should be performed before the femoral incisions, if possible, to prevent proximalization of the joint line.
The new patient-specific "simulation correction incision block" imitates the planned position of the definitive femoral component. In this way, the leg axis and the ligament tension in flexion and extension can already be checked after the tibial incision and corrected if necessary using the technique described below.
Knee prostheses with physiological kinematics
Around 1860, Themistocles Gluck still assumed a hinge mechanism in knee joints. It was not until the 1960s that this opinion was revised. Anatomical studies of the knee joint and in particular the cruciate ligaments led to the theory of the four-link joint. J-shaped femoral components were used in an attempt to mimic the physiological rollback motion. CT and uoroscopic studies finally showed that the medial knee joint component behaves more like a ball and socket joint, while laterally a.p. translation ends. The knee joint pivots during flexion-extension motion about an axis through the medial joint portion. Further cadaveric studies also showed that the distance between the permanent flexion axis of the femur and the distal and posterior condylar axes are approximately equal. The knee system presented here is based on these principles (ADVANCE Medial-Pivot Knee System, Microport). The femoral component has a constant sagittal radius in the range from 90° flexion to full extension. To allow physiological pivoting of the knee joint in a "fixbearing" system (fixed inlay), the medial inlay has an ultra-congruent shape to the femoral component (ball-in-socket) with a robust anterior edge. Laterally, the inlay's axial shape allows for 15 degrees of arcuate translation. Furthermore, medially, the femur remains in the same anteroposterior position during flexion as in a "posterior-stabilized" knee prosthesis. The medially ultra-congruent design provides a sense of stability throughout the entire range of motion. The risk of a "spin-out" is also eliminated thanks to the fixed inlay. By following this shape principle, the physiological kinematics of a healthy knee joint can be imitated.
