Abstract
In general, “alignment” refers to the position of all components in three dimensions. This discussion is limited to “varus-valgus” (v-v) alignment, or angulation in the frontal plane. This is largely determined by rotational position of the tibial and femoral components about the “z” (antero-posterior) axis. The earliest paper to note the importance of alignment, described only “valgus” on short x-rays.
It is difficult to argue that knee alignment is irrelevant, as angulation increases, so does the lever arm at the knee. (Biomechanics) Alignment is relevant to the development of osteoarthritis, most likely as the result of load on the medial compartment with varus alignment. (Natural History of OA) The first knee arthroplasties were an attempt to resurface worn cartilage with biological tissue and then non-biological material. Alignment and biomechanics were not considered. (History)
Frontal plane alignment can be depicted as “anatomic” (the angle between the femoral and tibial canals) on short radiographs or “mechanical” on full-length radiographs. Mechanical alignment may be reported in degrees or distance. Degrees describes the angle between a line from center of femoral head to center of knee (MA of the femur) with the line from center of knee to center of ankle (MA of tibia.) Alternately if a line is drawn form the center of the femoral head to the center of the ankle, MA may be described as the distance from the center of the knee to this axis along the joint line. Sometimes this is depicted by sectors in the knee. “Mechanical alignment” is a method of describing the angulation of the knee, not strictly a surgical technique.
Long radiographs: Short radiographs, though perhaps cost effective in most clinical settings, are unreliable images for studies of alignment. Physiologists and astute surgeons have always considered the entire limb. When clinicians applied full-length radiographs to clinical practice, along with navigation technology, assumptions evolved about the most desirable alignment of an arthroplasty. Arbitrarily, a neutral mechanical axis, or a straight line from the centers of the hip, knee and ankle was promoted. This also means that a line from hip to ankle would pass through the center of the knee. Many surgeons have entertained the erroneous concept that a “neutral mechanical axis” represents “normal human alignment: it does not. Others contend that a neutral mechanical axis necessarily means that there will be equal load on medial and lateral compartments: it will not.
Received wisdom about the necessity of a neutral mechanical axis has been questioned and yet malalignment and pre-operative deformity both appear to contribute to failure.
Stability is clearly important, because it limits deviations in alignment, and a range of alignments are probably highly functional, just as the knee may be loaded in a variety of directions. Dynamic features of patient activity are undoubtedly important, as is pre-operative deformity.
