Principles of Deformity Correction

Lower Limb Alignment and Joint Orientation

Bones, joints, and bone and joint segments can be two-dimensionally characterized using axis lines. A mechanical axis line connects the center of a proximal joint to the center of a distal joint ( Fig. 70.1 ). An anatomic axis line is the middiaphyseal line (see Fig. 70.1 ). Whereas the anatomic axis is used in the frontal and sagittal planes, the mechanical axis is used only in the frontal plane. The orientation of each joint can be measured between frontal- and sagittal-plane joint orientation lines and the mechanical and anatomic axes ( Fig. 70.2 ). The mechanical and anatomic axes of the tibia are parallel to each other (see Fig. 70.1A ). The tibial anatomic axis is normally a few millimeters medial to the tibial mechanical axis. In the femur, the mechanical and anatomic axes are convergent with each other (see Fig. 70.1B ).

The mechanical axis of the lower limb extends from the center of the hip to the center of the ankle ( Fig. 70.3A ). In a normally aligned limb, the mechanical axis of the lower limb passes through or slightly medial to the center of the knee joint line. The distance between the mechanical axis of the lower limb and the center of the knee joint is called the mechanical axis deviation (MAD; see Fig. 70.3A ). Deformities of the femur and tibia in the frontal plane lead to a MAD outside this normal range. Medial and lateral MADs can lead to arthrosis of the knee joint. To determine objectively whether MAD results from femoral or tibial deformity, the mechanical joint orientation angles (i.e., lateral distal femoral angle and medial proximal tibial angle) are measured and compared with the normal range (85 to 90 degrees; see Figs. 70.3B and C ). This assessment is called the malalignment test. Values outside the normal range indicate the source of MAD to be femoral, tibial, or both. When the femoral and tibial joint orientation lines in the frontal plane are not parallel to each other, this joint line convergence angle is another source of MAD (see Fig. 70.3D ). Angular deformities near the hip and ankle have little effect on MAD ( Figs. 70.4 and 70.5 ). It is therefore important to check the orientation of the ankle and hip to the individual bone mechanical or anatomic axis in the frontal plane. The orientation of the ankle and hip can be checked relative to the mechanical axis line in the frontal plane and the overall anatomic axis line, a line between the normal intersection points of the anatomic axis with the proximal and distal joints, in the sagittal plane. Alternatively, it can be checked relative to a segmental anatomic or mechanical axis line. Both assessments are called malorientation tests.

Characteristics of Deformity

Level of Angulation

Angular deformity leads to a bend or break in the anatomic and mechanical axis lines ( Fig. 70.6 ). The intersection point of the proximal and distal axis lines is the center of rotation of angulation (CORA; Fig. 70.7 ). Multiple levels of angulation have one CORA for each apex. The CORA can be found by drawing proximal and distal mechanical or anatomic axis lines. The anatomic axis line method of planning is illustrated in Figs. 70.8 to 70.11 . The mechanical axis method of planning can also be used and is explained in other publications. Anatomic axis planning can be used in the frontal and sagittal planes and easily identifies single-level or multilevel angular deformities (see Figs. 70.8 to 70.11 ).

Key Point

• The intersection point of the proximal and distal axis lines is the center of rotation of angulation (CORA).

Osteotomy Rules

The relationship of an osteotomy to the CORA determines the effect of an osteotomy on limb alignment. A line passing through the CORA dividing the transverse angle into two equal parts is called the transverse bisector line ( Fig. 70.12A ). Each point on this line can be considered a CORA ( Fig. 70.12B ). When the osteotomy plane passes through the convex cortex CORA and an opening wedge angular correction of the magnitude of deformity is performed, the proximal and distal anatomic and mechanical axes of the bone become colinear, and normal joint orientation is restored between the distal and proximal joints of that bone. The same correction is achieved when a closing wedge osteotomy of the magnitude of angulation converges on the concave cortex CORA. Both are examples of osteotomy rule 1 ( Fig. 70.13 ).

If the opening and closing wedge osteotomies are made at a level different from that of the CORA, the proximal and distal axis lines are realigned in angulation but become translated to each other (osteotomy rule 3; Fig. 70.14 ). Osteotomy rule 3 demonstrates that angulation through the osteotomy with an angulation correction axis (ACA) different from the CORA will correct an angular deformity but produce translation of the distal segment's axis as it becomes parallel to the proximal segment. This condition is called a secondary translation deformity. To avoid secondary translation deformity when the osteotomy is performed at a level different from that of the CORA, the osteotomy line can be angulated to straighten the bone and translated to correct the amount of secondary translation expected (osteotomy rule 2; see Fig. 70.14 ). The bone axes are realigned in a colinear fashion, and the joint orientation of the distal to the proximal joint is normal.