TKA: Balance Gaps

 

After the flexion gap has been established and the appropriate size femoral component applied, extend the knee. A symmetrical and balanced extension gap should be created.  This is sometimes difficult as it often requires elevation or lowering of the joint line. The patella helps determine the appropriate position of the joint line.

 

 

 

 

Tight

OK

Loose

Tight

1

2

3

OK

4

5

6

Loose

7

8

9

It is important to remember that adjustments to the femoral side of the arthroplasty can affect the knee in either flexion or extension, while  any change to the tibia affects both flexion and extension. This is part of the rationale for reconstructing the tibial side first. The following matrix (Fig. 79) suggests the nine situations that can occur during a trial reduction in a revision knee. It is worth reviewing these options and some of their potential solutions.

1. If a knee is too tight in both flexion and extension, reducing the height of the tibial articular surface maybe sulkient to balance the construct.

2. If the knee is tight in flexion but acceptable in extension, two options exist. An augment may be used with the distal femur. This will drop the joint line lower, and allow the use of a thinner tibial component. Another option is to use a smaller femoral component.

3. If the joint is loose in extension and tight in flexion, augmentation of the distal femur should provide a good arthroplasty with a thinner polyethylene component if the joint line is at its proper location. Another option is to use a smaller femoral component possibly with a thicker polyethylene component.

4. If the joint is acceptable in flexion but tight in extension, several options exist. One is to release the posterior capsule from the femur. Another alternative is to resect more distal femoral bone. This moves the femoral component proximally on the femur at the expense of elevating the joint line.

5. Obviously, if both components are acceptable, no further modification is necessary.

6. If the joint is acceptable in flexion and loose in extension, the probable solution is augmentation of the distal femur while using the same polyethylene component. This will drop the joint line and tighten the extension gap. Another option is to downsize the femoral component and use a thicker polyethylene component. This will probably raise the joint line.

7. If the joint is loose in flexion and acceptable in extension, a larger femoral component, may suffice. A second option is a thicker tibial articular surface with a more proximal femoral position.

8.  If the joint is loose in flexion and acceptable in extension, increasing the femoral size may equalize the gaps. Alternatively, moving the femoral component proximally and applying a thicker tibial articular surface will equalize the gaps. As a less desirable solution, one may choose to accept this situation if it is only of a mild degree, particularly in a highly constrained component.

9. If the joint is symmetrically loose in both flexion and extension, a thicker tibial articular surface will usually solve both problems.

In situations where two options exist to help solve the soft tissue mismatch, the position of the patella or the joint line help the surgeon decide which option to select.

Note: After applying one of these solutions, perform another trial reduction. This will identify any new problem or a variation of the initial problem that now may exist.