Abstract
Instability currently represents one of the main causes of residual pain and symptoms following TKA and thus is a major cause of revision total knee replacement, second only to component loosening in some series. Instability related to ligamentous laxity can be categorised by the pattern of relative laxity of the soft tissue structures and this in turn helps in determination of the bony alignment issue, component sizing or positioning problem or ligamentous abnormality that may be contributory and require correction. Instability patterns associated with TKA can be symmetrical and global type instability where there is laxity in all planes, and can also more commonly be asymmetrical or isolated laxity problems where there is good stability in some planes or positions of the knee but excessive laxity in at least one direction. Isolated laxity problems can be subcategorised into one of 3 patterns: Extension instability, Flexion instability, and Recurvatum. Global laxity can occur due to inadequate tibial component thickness, or globally incompetent soft tissues, and can present initially after TKA or alternatively can present late from slow stretch of soft tissues over time as can be seen with some pathologic states.
Asymmetrical or Isolated laxity occurs in the sagittal plane when medial vs. lateral “gaps” are unequal and may be due to contracture of tight structures either medially or laterally or can be due to insufficiency or injury of the ligamentous structures on one side vs. the normal structures on other side. Occasionally there is a combination of both contracture on one side and attenuation/stretch on the other side as seen in some patients with severe long standing genu varum or genu valgum. Asymmetrical laxity in the frontal plane generally results in unequal extension vs. flexion “gaps”. This can cause either anteroposterior laxity in flexion but full extension with good stability or alternatively, there may be AP stability in flexion but a lack of full extension in the presence of the exact same pattern of imbalance when a “too thick” polyethylene insert is used to correct what would otherwise be flexion instability. In both cases, the extension gap is tighter than the flexion gap. Isolated recurvatum occurs when the posterior capsular structures are relatively lax or deficient so that a knee that is otherwise stable in the medial-lateral plane in extension, and is stable in the AP plane when in flexion, hyperextends in the fully extended position. In any TKA procedure (but especially revision for instability) it is critical to understand the effect of selected bone resection (or build ups) on soft tissue balancing in order to avoid or treat ligamentous laxity: distal femur – effects extension gap only; posterior femur – effects flexion gap only; proximal tibia – both flexion and extension spaces.
During revision for instability, careful evaluation of the cause of the laxity and failure is critically important, especially if there is associated axial deformity or malalignment which generally must be corrected for any reconstruction or revision components to work. Most knees revised for instability issues will require a posterior stabilised or constrained condylar design. Constrained condylar implants are used to compensate for residual medial-lateral imbalance still present after standard soft tissue releases medially (subperiosteal tibia) or laterally (vis selective pie-crust method). However, if the patient displays residual major medial-lateral or global instability that cannot be corrected, or when there is an excessive flexion gap that cannot be stabilised with maximal allowable component sizing, a rotating hinge constrained total knee replacement design may be required. Recent data has shown that rotating hinges can work reliably in restoring stability to the knee in such cases with satisfactory durability and clinical results over time.
