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Mechanical Malalignment of the Knee Joint: How and When to Address
Introduction
Complex knee injuries are an orthopaedic problem that require knowledge not only of soft tissue anatomy but also lower limb biomechanics and alignment to establish an appropriate management plan and optimal patient outcome. Malalignment of the lower extremities causes eccentric redistribution of normal stresses in the joints. This eccentric stress causes degeneration of cartilage and subchondral bone. Additionally, alignment of the knee has significant long-term effects on the outcomes of reconstructive surgeries, including soft tissue reconstruction, osteotomies and arthroplasty. This chapter aims to discuss lower limb alignment in patients with compartment overload and in patients with chronic ligamentous injuries. The authors provide an overview of the normal lower limb alignment and biomechanics, coronal and sagittal malalignment, how to adequately assess alignment, decision making and osseous procedures to assess ligamentous patholaxities.
Normal Knee Alignment
In the coronal plane the anatomical axis is determined in relation to the intramedullary canals of femur and tibia to the centre of the knee ( Fig. 5.1A ). , The mechanical axis of the femur is a line from the centre of the femoral head to the midpoint of the knee. The mechanical axis of the tibia is a line from the midpoint of the knee to the centre of the tibiotalar joint ( Fig. 5.1B ). On anteroposterior (AP) evaluation the anatomical and mechanical axes of tibia coincide, whereas in the femur it makes an angle of 5 to 7 degrees. The normal lower limb in adults while weightbearing is 1 to 2 degrees of varus. ,
The mechanical axis of the lower extremity as a whole passes from the centre of the femoral head to the centre of the tibiotalar joint ( Fig. 5.1C ). This line determines the load travelling across the knee joint. In normal individuals the position of the mechanical axis passes slightly medial to the medial tibial eminence while weightbearing. This causes approximately 75% of the knee joint load to pass through the medial compartment in normal single-leg stance.
In the sagittal plane the tibial slope is measured as the angle between a line perpendicular to the mid-diaphysis of the tibia and the posterior inclination of the tibial plateaus ( Fig. 5.2 ). , The average adult tibial slope ranges from 0 to 18 degrees, with variations from side to side. ,
Biomechanics of the Knee
The peak force going through the knee joint is three times body weight during mobilisation. During the stance phase of gait, the supporting foot is placed nearer to the line of action of body weight. This more central positioning places the hip and tibia in an adducted position in the coronal plane. The equal force vector that arises from the ground reaction force (GRF) passes medial to the knee joint, creating an adduction moment, with a resultant compressive force through the medial compartment. The compressive force through the medial compartment creates tension through the lateral soft tissue structures. Evidence has shown that the rate of loss of articular cartilage and the progression to osteoarthritis correlate directly with the peak knee adduction moment during gait. Increasing varus alignment by 1 degree can increase the annual loss of cartilage by 0.44%. Further loss of medial articular cartilage continues to accentuate varus malalignment creating a larger adduction moment and resulting in a vicious cycle of increasing medial compartment loading and worsening articular cartilage wear. ,
Coronal Malalignment
Varus
Primary varus is described as varus that occurs with loss of the medial meniscus and damage to the articular cartilage in the joint. Double varus results from the tibiofemoral osseous alignment described earlier with associated separation of the lateral tibiofemoral compartment because of deficiency of the lateral soft tissues. Triple varus then occurs in the setting of more significant injury, leading to separation of the lateral tibiofemoral compartment and increased external tibial rotation and hyperextension. Additionally, a dynamic varus gait is described to accompany this group of pathological conditions. This is referred to as a varus thrust gait . A varus thrust is the dynamic lateral movement of the knee during stance phase in ambulation, with the return to a less varus alignment during the swing phase.
Varus alignment has also been shown to increase the load on the anterior cruciate ligament (ACL). A cadaveric study showed increased stresses through the ACL with sequential varus loading of the knee. The study demonstrated that strain was significantly higher in the varus loaded knee (53 N) compared with a neutrally loaded knee (31 N). A similar study demonstrated that the forces on the ACL graft significantly increase after sequential sectioning of the lateral (fibular) collateral ligament (LCL), popliteofibular ligament (PFL) and popliteus tendon (PT). Forces were noted to be significantly higher after LCL transection during varus loading. Additionally, gait studies have reported increased load on the lateral soft tissue structures with separation of the lateral tibiofemoral joint and ‘condylar lift-off’ during the stance phase and an increased medial joint compartment pressure. Thus untreated posterolateral corner (PLC) injuries contribute to cruciate ligament graft reconstruction failure by allowing significantly higher forces to stress the graft with varus loading at varying degrees of flexion. Studies investigating the causes of ACL and posterior cruciate ligament (PCL) reconstruction failure have both noted the presence of varus malalignment as a risk factor for graft failure. , In summary, failure to address the osseous malalignment in this context will result in excessive strain on the reconstructed soft tissues and continued pathological loading of the medial joint compartment and chondral injury.
Valgus
Valgus malalignment with concomitant instability is less common than in varus instability. It is defined as a weightbearing line crossing the lateral tibial eminence towards the lateral compartment or greater than 10 degrees or valgus malalignment of the mechanical axis in the frontal plane. This malalignment has been shown to put excess strain on the medial-sided structures of the knee. Like the varus hyperextension thrust described earlier, a phenomenon may be seen in the valgus knee with associated medial soft tissue deficiency. This phenomenon, however, is significantly less common because of the medially placed centre of mass and resultant adduction moment. The valgus deformity must be dramatic before the medial soft tissues are compromised.
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