Indications

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  Monocompartmental arthritis of the knee

Contraindications

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  Relative contraindications to the Coventry osteotomy include lateral collateral ligament instability, lateral subluxation, medial plateau depression, knee flexion less than 90 degrees, knee flexion contracture greater than 10 degrees, lateral compartment arthrosis, advanced age, and obesity.

Classification

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  This is based on the type of deformity present and commonly includes a combination of two or more deformities.

  
  
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  Classification is divided into bone deformity (varus, valgus, recurvatum, procurvatum, torsion, limb length discrepancy) and joint deformity (lateral collateral ligament laxity, medial collateral ligament laxity, plateau depression, lateral subluxation, patellar maltracking, flexion contracture).

Surgical Technique

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  Customized osteotomy allows an “á la carte” approach to each individual patient and designs the osteotomy technique to specifically address the deformity or deformities present.

Biomechanics

Static malalignment is readily documented on long standing radiographs by measuring the mechanical axis deviation (MAD) in the frontal plane. Measurement of joint orientation angles and joint line convergence identifies the origin of the MAD (femur, tibia, joint line convergence). These objective parameters are not always a reliable means of predicting outcome after corrective osteotomy. Theoretically, the moment arm created by the medial location of the ground reaction force vector is a better objective parameter to determine medial compartment loading. Unfortunately, this vector cannot be readily determined except in a gait laboratory. Gait studies of the knee have shown that the predicted load on the medial compartment is related to alignment. Hsu et al. reported that the load on the medial compartment in a normally aligned knee is approximately 70%, compared with 30% on the lateral compartment ( Fig. 4-1 ). Furthermore, they showed that with an increasing varus tibiofemoral angle, the load on the medial compartment increased to 100% when the knee was in 6 degrees of mechanical varus. Based on their model, “valgusizing” the knee to 4 degrees changes the loading pattern to 50:50. If the load on the knee were 70% in the normal situation, everyone would likely have medial compartment osteoarthritis. However, the model presented by Hsu et al. does not factor in joint reaction forces from the surrounding muscles and ligaments. Maquet proposed that the tensor fascia lata (TFL) and gluteus maximus equalized the forces around the knee through their pull on the iliotibial band, neutralizing the adductor moment arm ( Fig. 4-2 A ).

The medial plateau force is 70% in single limb stance when the mechanical axis passes through the center of the knee in a normally aligned knee. It is 95%, with only 6 degrees of mechanical tibiofemoral varus and is reduced to 50% with 4 degrees of valgus and to 40% with 6 degrees of valgus.

(Reprinted with permission of Lippincott Williams & Wilkins from Hsu RW, Himeno S, Coventry MB, Chao EY: Normal axial alignment of the lower extremity and load-bearing distribution at the knee. Clin Orthop Relat Res 255:215–227, 1990.)

A , Static analysis predicts 70% of the load will pass through the medial compartment in a normally aligned knee. Dynamic analysis factors in the pull of a strong tensor fascia lata (TFL) and predicts equal balance of the load through the medial and lateral compartments. B , When genu varum is present, the load on the medial side is predicted to be 90%, but if TFL dynamic pull is present, it is reduced to 50:50. C , When lateral collateral ligament (LCL) laxity is present, a medial thrust results, especially if the TFL is weak. If a strong TFL is present, it can reduce the medial compartment forces. GRV, ground reaction vector.

[ A , Reprinted with permission from Mont MA, Stuchin SA, Paley D, et al: Different surgical options for monocompartmental osteoarthritis of the knee: High tibial osteotomy versus unicompartmental knee arthroplasty versus total knee arthroplasty: indications, techniques, results, and controversies. In Helfet DL, Greene WB (eds): Instructional Course Lectures 53. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2004, pp 265–283. B, C , Copyright 2007 Rubin Institute for Advanced Orthopedics.]

Using cadaver and magnetic resonance imaging measurements, investigators at the Oxford Orthopaedic Engineering Center developed an anatomy-based mathematical model to predict loads transmitted across the knee. This model incorporated the lines of action and moment arms of the major force-bearing structures crossing the human knee joint, including both muscles and ligaments. Theoretical values derived from this model replicate the previously published experimental measurements presented by Herzog and Read, which validates their model. Including contributions from muscles and ligaments, both experimentally measured and theoretically calculated forces across the knee are more evenly distributed than published results have suggested. The difference between the static single limb standing simulations and those that factor in the surrounding muscle forces is mostly attributable to the pull on the iliotibial band by the TFL and the gluteus maximus muscles. In a well-conditioned person, these muscles counter the adduction moment arm on the knee, unloading the overloaded medial side and transferring that load to the lateral side. As one gets older (older than 35 years) and naturally loses muscle mass and strength, the protection afforded the medial compartment by these muscles is diminished and lost ( Fig. 4-2 B,C ). The loss of protein can precipitate the progressive deterioration of the medial compartment that most commonly occurs in people older than 40 years, which has led us to prescribe gluteus maximus and TFL strengthening exercises to treat early medial compartment osteoarthritis (e.g., 45-degree oblique straight limb-raising exercises). Therefore, in persons older than 40 years, the static model presented by Hsu et al. becomes representative of the clinical situation.

The dynamic loads that occur during walking and other weight-bearing activities of daily living have been difficult to accurately determine. Important issues regarding the dynamics of knee malalignment have been reviewed in detail by Andriacchi. The normal forces that act on the lower extremity during gait produce moments tending to flex, extend, abduct, and adduct the knee. These are the primary factors influencing the distribution of medial and lateral loads across the knee. The ground reaction force acting at the foot during the stance phase of gait passes medial to the center of the knee. The perpendicular distance from the line of action of this force to the center of the knee is the length of the lever arm for this force. The product of the magnitude of the force and the length of the lever arm results in an adduction moment acting on the knee. This adduction moment during gait is an external load tending to thrust the knee into varus; it is also known as a lateral thrust.

The external forces and moments acting on the lower extremity can be measured directly in a gait laboratory. The internal forces acting through muscles, through ligaments, and on joint surfaces are of greater interest but can only be estimated based on the external forces and moments measured. Mechanical equilibrium mandates that external forces acting on the limb must be balanced by internal forces generated by muscles and ligaments. Prediction of internal forces is extremely complicated because of the many combinations of muscle and soft tissue forces that can balance the external forces and moments acting on the limb. Solving this problem requires several simplifying assumptions, the most basic of which is to group internal structures together. Analysis of the relationship between external loads and internal forces under these assumptions allows estimation of the magnitude of the joint reaction force acting across either the medial or lateral compartment independently. The distribution of the medial and lateral joint reaction forces shows that the adduction moment is the primary factor producing the higher medial joint reaction force during normal function. For a group of normal participants, the maximum joint reaction force across the knee is approximately 3.2 times the body weight, with 70% of the load passing through the medial compartment. The average maximum magnitude of the adduction moment during normal gait for this population has been calculated as approximately 3.3% of the product of body weight and height. This adduction moment is greater than the moments calculated for either flexion or extension of the knee in the same study group.

Some patients modify their gait, effectively reducing the load on the medial compartment of the knee. The adaptive mechanism used reduces the adduction moment and has been related to a shorter stride length and an increase in external rotation of the foot (toe-out position) during stance phase. The toe-out position places the hindfoot closer to the midline, beneath the center of gravity. This simply moves the ground reaction vector toward the center of the knee, effectively reducing the lever arm of the external ground reaction force and therefore the resulting adduction moment. Patients are considered to have high adduction moments if the calculated moment exceeds 4% of the product of body weight and height when walking at speeds of approximately 1 meter per second.

The clinical outcome after treatment of patients with varus gonarthrosis by valgus high tibial realignment osteotomy has been closely related to the magnitude of the adduction moment measured during preoperative gait analysis. Patients who had low preoperative adduction moments had better clinical results initially, and the results were sustained during a mean follow-up period of 6 years. The valgus correction was maintained with follow-up in 79% of the low adduction moment group compared with only 20% of the high adduction moment group.

Load transmission across the knee can effectively be altered by adjusting the location of the center of gravity. This dynamic compensation involves either the use of external support or gait modification. Shifting the upper body center of mass to a position directly over the involved limb can decrease the medial compartment force by 50% compared with its value when the center of gravity is positioned in the midline. Clinical evidence has already established the importance of gait alteration and its relationship to results after corrective high tibial osteotomy (HTO). Patients with the best clinical outcomes are able to modify their gait, externally rotating the limb and developing a lower adduction moment at the knee.

Joint laxity is a further confounding variable to consider when determining the risk of developing osteoarthritis secondary to malalignment (see Fig. 4-2 C ). Sharma et al. reported that ligament laxity can precede the development of osteoarthritis. Ligament laxity can result in dynamic malalignment during gait, with associated changes in loading patterns across the knee. Collateral ligament laxity can increase the risk of gonarthrosis and cyclically contribute to progression of the disease. Lateral collateral ligament (LCL) laxity typically is associated with varus malalignment and, when superimposed, might have a synergistic effect. The TFL can protect the knee from overload caused by LCL laxity. Again, this protection is gradually lost or overwhelmed with increasing age, deconditioning, and deformity.

Historical Background

The concept of using HTO to treat monocompartmental osteoarthritis is credited to Jackson and Waugh, who presented a report of eight procedures in 1961. The authors performed an osteotomy distal to the tibial tuberosity; both closing wedge and concave distal dome osteotomies were described. Difficulties with bone healing in the subtuberosity region led Coventry, in 1965, to present a report about closing wedge osteotomy proximal to the tuberosity through cancellous bone. Maquet presented a report of a concave distal dome osteotomy. The Maquet osteotomy was designed to take advantage of the rapid metaphyseal bone healing of the region above the tuberosity and to add an element of adjustability.

The common goal for all the HTO procedures was to shift the mechanical axis from the medial compartment to the lateral compartment. Although it is impractical to completely unload the medial compartment, the goal of HTO is to reduce the load on the medial compartment. In the normally aligned knee (2 degrees of tibiofemoral mechanical varus), the medial compartment has been estimated to take 75% of the load during single limb stance. When the mechanical axis passes through the center of the knee, the medial compartment bears 70% of the load. When the mechanical axis is moved into 4 degrees of valgus, the load is 50% medial and 50% lateral. When the mechanical axis is moved into 6 degrees of valgus, the load is 40% medial and 60% lateral (see Fig. 4-1 ). Most authors recommend that for treatment of monocompartmental osteoarthritis, the mechanical alignment of the lower limb should be moved into 2 to 6 degrees of mechanical valgus. Hernigou et al. showed that the best results were with 3 to 6 degrees of mechanical valgus and that results deteriorated when the mechanical valgus was more than 6 degrees. Fujisawa et al. recommended that the mechanical axis pass between 30% and 40% lateral to the center of the tibial spines. This distance has been termed the Fujisawa point ( Fig. 4-3 ). Jacobi and Jakob modified the overcorrection recommendation made by Fujisawa et al. based on the amount of cartilage space remaining on the medial side.

Fujisawa et al. divided the medial and lateral plateaus by the percentage of distance from the center of the knee. The medial and lateral edges of the medial and lateral plateaus were considered to be 100%, and the center of the knee was considered to be 0%. The best results from high tibial osteotomy (HTO) were obtained when the mechanical axis line of the limb passed through the 30%-to-40% lateral plateau region. We call this the Fujisawa point.

(From Paley D: Principles of Deformity Correction. Berlin, Springer-Verlag, 2005.)

The Coventry procedure has become the “knee-jerk” response to monocompartmental osteoarthritis. Conversions of previous Coventry osteotomies to TKR have been associated with poor results. Numerous factors contribute to greater technical difficulty and possibly poorer results of TKR after Coventry osteotomy. Because bone is resected proximal to the tibial tuberosity, the tuberosity moves closer to the knee joint line. After the osteotomy, the patella might ride proximally, creating a pseudo-patella alta. It is a “pseudo-alta” because in cases of true patella alta, the patellar tendon is abnormally long, whereas in this case, it is of normal length. Alternatively, the patella might not be able to ride proximally because of the tethering retinaculum. The patellar tendon scars down and contracts, especially if the knee is splinted in extension after the osteotomy. This leads to a pseudo-patella baja according to the Insall ratio. Again, this is a “pseudo-baja” because the tibial tuberosity–to tuberosity–patellar distance decreases, although the level of the patella to the femur remains the same. After TKR in the case of patella alta, the thickness of the tibial prosthesis restores the level of the tibial tuberosity and thereby pulls the patella down to the normal level by means of the contracted shortened patellar tendon. Eversion of the patella for exposure is more difficult with pseudo-patella baja. Bone resection with the wedge based laterally leads to truncation of the proximal tibia. This leaves the lateral and posterior tibial plateau thin and unsupported. This scenario can make seating a large central or a peripheral tibial component peg problematic.

The valgus deformity resulting from the overcorrection created during osteotomy can also make TKR more difficult and might require greater bone resection. Soft tissue considerations, such as previous incision, previous peroneal nerve palsy, ligamentous laxity secondary to the osteotomy, and flexion deformity of the knee, all make TKR more difficult and complication-prone after previous Coventry HTO.

Bone Deformities of Femur and/or Tibia

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  Varus

  
  
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  Valgus

  
  
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  Recurvatum

  
  
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  Procurvatum

  
  
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  Torsion

  
  
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  Limb length discrepancy

Joint Deformities

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  LCL laxity

  
  
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  Medial collateral ligament (MCL) laxity

  
  
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  Plateau depression

  
  
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  Lateral subluxation

  
  
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  Patellar maltracking

  
  
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  Flexion contracture

Bone and joint deformities rarely occur in isolation. Common combinations include varus deformity plus fixed flexion deformity, varus deformity plus MCL pseudolaxity, varus deformity plus anterior cruciate ligament (ACL) or LCL laxity, varus deformity plus lateral subluxation, and varus deformity plus rotational deformity.

Varus Deformity Only

The level of osteotomy for the proximal tibia can be proximal or distal to the tuberosity. When performing the osteotomy, the level of the center of rotation of angulation (CORA) should be considered. The CORA is almost always at the level of the joint or just distal to the joint. Therefore, if the osteotomy is made proximal to the tuberosity, it requires only angulation. If the osteotomy is made distal to the tuberosity, it requires angulation and translation. Proximal to the tuberosity, a closing wedge osteotomy narrows the distance between the joint line and the tibial tuberosity, making future TKR more difficult. To preserve the distance from the tuberosity to the joint while still performing a closing wedge osteotomy at the Coventry level, the osteotomy can include the tuberosity with the proximal segment. Opening wedge osteotomy proximal to the tuberosity tightens the MCL, which often has pseudolaxity from loss of medial joint space cartilage. An opening wedge osteotomy requires either bone graft for acute corrections or gradual distraction by an external fixator for bone regeneration. Hernigou et al. published a very large series with a transverse medial opening wedge osteotomy. Franco et al. modified this osteotomy, starting more distal on the medial side and ending just proximal to the head of the fibula, leaving the lateral cortex intact. Their opening wedge step plate supported the base of the osteotomy only as long as the lateral cortex remained intact. Staubli et al. modified the osteotomy of Franco et al. and developed a medial opening wedge locking plate for fixation. Jacobi and Jakob also reported on the technique of Staubli et al. Franco et al. recommends bone grafting the defect if the base of the wedge is more than 10 mm, whereas Staubli et al. almost never graft the defect. With a locking plate, one need be less concerned regarding maintaining the integrity of the lateral cortex because of the fixed angle relationship of the screws to the plate ( Fig. 4-4 ). Osteotomies distal to the tuberosity need to be performed in combination with lateral translation because they are far from the CORA ( Figs. 4-5 and 4-6 ). This applies equally to both opening and closing wedge osteotomies. Because of the poorer healing potential of this region, it is essential to preserve the periosteum and preferably perform the osteotomy with a minimally invasive approach. Bony contact at the osteotomy site is greater than for an opening wedge osteotomy because translation inserts the corner of the proximal segment into the medullary canal of the distal segment. Dome osteotomy is also an angulation-translation correction. The Maquet dome osteotomy consisted of concave distal rotation around an axis in the center of the circular cut distal to the CORA. The Maquet dome therefore creates a medial translation deformity. Paley et al. and Paley described the focal dome osteotomy ( Fig. 4-7 ) with which the center of the circular cut is “focused” on the CORA. This concave distal dome osteotomy translates in the same direction as the straight angulation-translation osteotomy and is distal to the tuberosity.

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