All cases of patellar instability, dislocation or subluxation, and most of patellofemoral pathologies result from of an excess of force being applied to the patella. This force is produced principally by the quadriceps muscle.

The magnitude of the quadriceps force needed to control the body’s mass passing through the bending knee is determined by the body mass, the length of the femur and tibia that creates leverage, and the degree of knee flexion. Brattström in 1964 referred to this as the quadriceps resultant.¹ The direction of this resultant is largely influenced by the alignment of the limb. Because the quadriceps is attached to the patella, any change in the direction of the quadriceps resultant force directly affects the force distribution in the patella-trochlear joint.

The geometry of either the femur or the tibia or both may vary from normal, causing the knee joint to lose its normal and optimal orientation in space. This alters the direction of the quadriceps vector and the location of the weight transfer through the knee joint.

A skeletal abnormality may occur in the coronal (frontal) plane or in the transverse (horizontal) plane or both. Because this force from the quadriceps is responsible for nearly all patellofemoral joint pathology, it may be crucial to change this force by osteotomy when abnormal limb alignment exists. Osteotomy, because it changes the direction of the quadriceps resultant, is a powerful tool in the treatment of patellar instability.

When the limb is viewed in the coronal plane, a normal tibiofemoral angle of approximately 5.8° to 6° exists between the tibial and femoral shafts.² Because the femoral head is medial to the proximal femoral shaft, a line from the center of the femoral head center to the ankle center passes just medial to the center of the tibial spines (Figure 32.1).

This line, now called the mechanical axis, was described by Jan Mikulicz-Radecki in 1879.

This line approximates the line of weight transmission through the extended knee. This mechanical axis is inclined medially about 3° because the feet are normally closer to the midline than the hip joints. With 3° medial inclination of the limb, a horizontal joint line requires the angle between the joint line and the limb mechanical axis be 87° on the medial proximal tibia and 93° between the distal femur articular surface and the mechanical axis of femur on the medial side. These angles
are used to determine whether angular deformities in the coronal plane are located in the femur or in the tibia.

Standing radiographs of the entire limb with full weight bearing and with the flexion-extension axis of the knee joint in the coronal plane must be obtained to make this determination.

The 87° angle between the axis of the tibial shaft and the top of the tibial plateau is sometimes called the medial proximal tibial angle.

The 93° angle between the mechanical axis of the femur (femoral head to middle of the knee) and the distal aspect of the femoral condyles is sometimes called the mechanical medial distal femur angle, whereas the lateral complementary angle, the mechanical lateral distal femur angle, equals 87°.

A medial proximal tibial angle less than 87° represents varus in the tibia. A medial distal femur angle less than 93° represents varus in the femur.

Paul Maquet³ explained that the resultant vector of weight transmission through the knee (R) is the sum of the body mass (P), which is located far to the medial side of the knee joint, and the total muscle force (L) crossing the knee joint. That is, R = P + L. A greater lateral vector (lateral muscle mass) is needed to balance the body weight vector placing the resultant (R) near the center of the knee joint (Figure 32.2).

The resultant vector of weight transmission closely approximates the mechanical axis. If the mechanical axis is more medial than normal, varus deformity exists in the limb and a higher percentage of the weight is transmitted medially. If the line is more lateral than normal, a valgus deformity exists in the limb and a higher percentage of the weight is transmitted laterally.

A knee joint may be in either more or less varus or valgus as a result of altered angulation of either the femur or the tibia. A greater valgus angle at the knee increases the lateral displacement force on the patella and a greater varus angle at the knee decreases the lateral displacement force. The lateral displacement force is responsible for lateral patellar dislocation. Reducing this force may be sufficient or essential in the treatment of lateral patellar dislocation.

Fujikawa⁴ in 1983 demonstrated that with increasing varus deformity of the knee, there was a marked shift in patellar contact from the lateral to the medial trochlea.

Madigan⁵ in 1975 noted that six of seven knees with genu valgum had failed results after surgery for patellar dislocation and genu valgum was present in 75% of all his surgical failures with proximal patellar realignment.

David Podeszwa⁶ and his associates at Texas Scottish Rite Hospital performed 10 varus osteotomies in 10 adolescents with genu valgum and recurrent dislocation of patella (average number dislocations per patient were 6.8). Trochlear dysplasia was obviously present in 8 of 10 patients. No surgery was performed on the patellofemoral joint. This resulted in eight patients without recurrent instability, one patient with one episode of patellar subluxation, and one case of redislocation leading to patellar stabilization surgery. This study was brilliant, in avoiding the confounding variables of additional patellofemoral joint stabilization surgeries, because it addressed only the valgus alignment deformity. It, therefore, underscores the importance of a neutral limb alignment for a proper balance of the patella on the femur.

Publications examining surgery for patellar instability that involve multiple concomitant procedures fail to provide evidence for which procedure is most beneficial. Decision making in a condition that is the result of multiple contributing variables is hampered when the relative importance of each variable is unknown.

Observational studies indicate neutral limb alignment is important to a balanced force application on the patella. Because the magnitude and direction of the quadriceps vector cannot be determined in the clinical setting, the exact contribution of a valgus deformity to patellar instability can only be guessed. Proof of the importance of neutral limb alignment for patellar balance must await new studies, which can correlate quadriceps force analysis with limb geometry.

Varus deformity may be associated with medial patellar dislocation, and a valgus limb is often associated with lateral patellar instability.

The valgus may exist in the femur or in the tibia or in both.

Osteotomy should be at the site of deformity as determined by assessing alignment on the full-length standing radiograph. Osteotomy should be performed on the bone with greatest deformity. There are instances where correction of both the tibia and the femur may be desirable.

Osteotomy correction may be with an angular wedge, a hemi wedge, or crescentic cut. Wedge osteotomy is performed most commonly.

For a valgus deformity, an open wedge may be used laterally, or a closing wedge may be placed medially.