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. ,




Fig. 5.1


(A) Anatomical axes of the femur and tibia. (B) Mechanical axes of the femur and tibia. (C) Mechanical axis of the lower limb.


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. ,




Fig. 5.2


Tibial slope.


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.


Sagittal Malalignment


The tibial slope contributes significantly to sagittal alignment and stability of the knee. Malalignment can result from either increased or decreased posterior tibial slope. Studies have shown there is an association between slope and tibial translation in the knee during weightbearing. Larger tibial slope results in greater anterior tibial translation. This has been shown in both the ACL-intact and ACL-deficient knees. , Increasing the posterior tibial slope has been shown to result in an increased shear on the ACL in the stance phase. A cadaveric study demonstrated that an increase of 4 degrees of posterior slope results in an increase in 3 mm of anterior translation in the resting position and 2 mm with axial compression. Similarly, a negative tibial slope places greater strain on the PCL because of posterior translation of the tibia and/or hyperextension. Thus this would suggest increasing slope may be beneficial in reducing tibial sag in the PCL-deficient knee but increases forces on the ACL. Multiple studies have implicated increased tibial slope as a risk factor for ACL injury and failure of ACL reconstruction. One such study demonstrated that patients with a tibial slope greater than 12 degrees had a 59% chance of further ACL injury compared with 23% in those with a slope less than 12 degrees. Similarly, decreased tibial slope has been implicated as a risk factor for isolated PCL injury.


Clinical Assessment


History


When conducting a clinical assessment, a thorough history focusing on the patient’s main complaint is elicited, which in the context of the complex knee injury will include instability and/or pain. A history of significant pain may be associated with chondral pathological conditions or degenerative changes, which should be taken into consideration if planning an alignment correction. Mechanical symptoms of catching or locking could indicate meniscal pathological conditions that should be addressed at the time of surgery. Descriptions of preexisting developmental problems, previous knee pathological conditions or surgeries should be elicited. Factors that may affect healing and postoperative rehabilitation should always be documented and optimised preoperatively. This includes a thorough social history (smoking, alcohol or substance abuse) and relevant medical problems such as diabetes mellitus. These factors have been associated with an increased rate of complications after osteotomy .


Physical Examination


Clinical examination begins with assessment of weightbearing alignment and gait abnormalities in both the coronal and sagittal planes. Abnormalities may be subtle, and appropriate attention should be given because varus, valgus or a hyperextension thrust may be present. It should be noted if walking aids and braces are being used. All deformities encountered are examined for their ability to be corrected. Muscle power and wasting are important because they can affect the outcome and rehabilitation. A thorough assessment of collateral and cruciate ligaments, including posterolateral, posteromedial, anterolateral and anteromedial corners, should be completed to assess soft tissue integrity. The presence of passive hyperextension will provide a clue to soft tissue deficiency (posteromedial or posterolateral corner laxity) or may also be related to abnormal tibial slope. A complete baseline neurovascular assessment is of paramount importance, including distal pulses, perfusion and common peroneal nerve function.


Radiological Assessment


Radiological investigation should be directed by the history and physical examination and will likely include basic radiographs and advanced imaging. Plain radiographs should include bilateral weightbearing AP views of the knee in full extension, a bilateral weightbearing posteroanterior view in 45 degrees of flexion (Rosenberg view) and lateral and skyline views ( Fig. 5.3A –C). Bilateral mechanical axis alignment views (hip–knee–ankle) are included after clinical assessment suggestive of malalignment in the coronal or sagittal planes ( Fig. 5.3D ). In the acute scenario, if significant coronal plane malalignment is suspected, a monopedal-stance hip–knee–ankle view of the contralateral limb may be used to determine the extent of deformity present. In the coronal plane, neutral alignment is defined as a line passing from the femoral head to the centre of the tibiotalar joint which passes between the tibial spines. Medial and lateral axis deviation is defined when that line passes medial or lateral to the tibial eminences, respectively. , The extent of deviation is determined by measuring the distance from the medial end of the tibial plateau and dividing it by the distance across the tibial surface. It is important to note that the knee must be in full extension and neutral rotation to accurately calculate deformity with this method. Flexion and rotation, and more dramatically the combination of both, can change the appearance of the mechanical alignment in the coronal plane on hip–knee–ankle radiographs. ,




Fig. 5.3


Radiographic assessment includes (A) bilateral standard anteroposterior (AP) view in full extension, (B) bilateral posteroanterior Rosenberg view, (C) skyline views of the patella, (C) horizontal beam lateral, and (D) bilateral hips to ankle AP weight-bearing view. These views together help identify joint line obliquity, posterior tibial slope and side-to-side differences in bony morphology. Stress radiographs are ordered as needed.


Stress radiographs provide an objective measure of the extent of injury in the coronal and sagittal planes. It is a useful adjunct for preoperative diagnosis and postoperative assessment of a successful radiological outcome. The contralateral side can be used as a control for comparison. These series include varus/valgus stress radiographs and kneeling PCL stress radiographs. ,


Magnetic resonance imaging (MRI) is performed routinely in both the acute and chronic setting. , MRI is used to assess the injured ligamentous structures and to rule out concomitant meniscal and chondral pathological conditions. Meniscal and chondral pathological conditions could be the result of initial injury or be sequelae of instability or abnormal biomechanics across the knee joint. , A computed tomography (CT) scan may be utilised in the presence of fracture deformity or rotational abnormalities that necessitate a rotational profile.


Choice of Realignment Procedures


The goal of realignment osteotomy is to correct the mechanical axis, which helps in neutralising the load going through the knee joint and at the same time reduces excessive stresses and strains on the soft tissue envelope of the knee. , In the context of ligamentous pathological conditions of the knee, this correction also neutralises the forces that would otherwise cause undue stress on reconstructed ligaments. The procedure performed is determined according to the thorough clinical and radiographical assessment. Coronal plane pathological conditions can be addressed on the tibial or femoral side with an opening or closing wedge osteotomy. Sagittal plane deformity is generally addressed on the tibial side because of its relationship with tibial slope.


In ligamentous deficiency with no previous chondral pathological condition, restoration of the mechanical axis to a point passing between the medial and lateral tibial eminences allows for more equal loading of the medial and lateral compartments and reduced strain through the associated soft tissue structures. If, however, chondral degeneration is already present, the degree of correction may be tailored to offload the affected compartment because the osteotomy also offers the benefit of reducing excessive loads that would accelerate cartilage wear. Importantly, care must be taken not to overcorrect the knee past the down slope of the appropriate tibial eminence. Preoperative planning of the osteotomy is completed using the method described by Dugdale et al. ( Fig. 5.4 ).


May 3, 2021 | Posted by in ORTHOPEDIC | Comments Off on Mechanical Malalignment of the Knee Joint: How and When to Address
Premium Wordpress Themes by UFO Themes