Extension Instability: Symmetrical

Symmetrical extension instability occurs when medial and lateral collateral ligament tensions are lax in extension. In this relatively rare situation, the combined thickness of the components is not sufficient to restore physiologic tension to the ligaments in extension only; this has been termed instability due to bone resection.^43,⁴⁴ Excessive bone removal from the femur will selectively increase the extension gap without altering the flexion gap; this is termed gap inequality.¹⁷

Management of isolated symmetrical extension instability caused by excessive femoral bone loss can be challenging. A thicker tibial insert will not solve the problem. Increasing the tibial insert thickness will elevate the joint line and excessively tighten the flexion space (Fig. 129-3).^37,⁴⁴ Elevation of the joint line can adversely affect knee kinematics (particularly the patellofemoral joint) and lead to worse clinical outcomes.³¹ Distal femoral augments should be used to move the joint line distally. Such augments are available in most contemporary revision total knee systems.

Figure 129-3 Excessive bone removal from the distal part of the femur cannot be managed with the use of a thicker tibial insert, which will elevate the joint line and excessively tighten the flexion space.

(Reprinted with permission from Vail TP, Lang JE: Surgical techniques and instrumentation in total knee arthroplasty. In Insall JN, Scott WN [eds]: Surgery of the knee, ed 4, vol 2, Baltimore, Md, 2006, Churchill Livingstone, pp 1455–1521.)

Extension Instability: Asymmetrical

Asymmetrical extension instability is much more common than symmetrical extension instability.³⁰ This situation is typically related to preoperative angular deformity of the knee and is caused by persistent or iatrogenic ligamentous asymmetry after the knee is replaced (Fig. 129-4).^29,^30,^37,⁴⁴ Fear of creating iatrogenic ligamentous instability in the opposite direction is a frequent reason for incomplete ligament balancing and undercorrection of deformity.

Figure 129-4 A, Example of a varus knee with a contracted medial collateral ligament that has been incompletely released. B, The medial collateral ligament remains tight, and the lateral collateral ligament is subsequently lax.

In the case of a varus knee, concerns about over-releasing the medial collateral ligament can leave the knee tight on the medial side—a situation exacerbated by leaving the limb alignment in varus.^29,^37,⁴⁴ Excessive medial collateral ligament tension and varus malalignment can result in medial overload with polyethylene wear and further stretching out of the lateral ligaments.^29,^37,⁴⁴ Recurrence of deformity may occur postoperatively. An appropriate medial release as originally described by Insall and associates should be performed when necessary.¹⁵ A thicker tibial insert may be required following medial release to restore tension in the collateral ligaments. Lateral collateral ligament advancement can be considered if complete medial release fails to balance the knee.¹⁸

Asymmetrical extension instability may result from undercorrection of a valgus knee deformity. Failure to appropriately release contracted lateral tissues will leave laxity or redundancy in the medial collateral ligament. The medial collateral ligament has no ability to tighten over time, and the valgus deformity will recur.^37,⁴³ Minimal laxity of the medial collateral ligament should therefore be accepted. Sequential lateral ligament release approaches^12,¹⁵ have been associated with frequent over-release of the lateral soft tissues.²⁷ As a result, the “pie-crust” multiple puncture technique has gained widespread use.^4,²⁷ To check for an over-released lateral side, the trials can be inserted and the knee placed in a “figure-of-four” position (90 degrees of knee flexion while the surgeon holds the foot and allows the hip and knee to maximally externally rotate). If the post of a posterior stabilized insert subluxates from the femoral housing, a thicker or more constrained tibial insert should be used.⁴

Complete correction of severe valgus deformity incurs the risk of peroneal nerve stretch.⁵ Satisfactory results have been reported with intentional deformity undercorrection and the use of a varus/valgus constrained implant to compensate for residual medial collateral ligament laxity.⁵ This approach carries the theoretical concern that a constrained implant will impart additional stresses to the implant and fixation interfaces and should be reserved for elderly, lower demand patients.

Intraoperative iatrogenic collateral ligament injury can occur, most commonly when the proximal tibia is cut. Vigorous attempts to assess stability intraoperatively can lead to ligament stretch or rupture, typically of the medial collateral ligament. Surgical reapproximation of the ligament is the recommended approach.²¹ The hamstring tendons may be used to augment the repair, and a condylar constrained implant may be used to enhance stability.^5,²¹ Postoperative bracing may help reduce stresses on the healing ligament.²¹

Flexion Instability: Symmetrical

Symmetrical flexion instability is likely under-reported as the diagnosis can be elusive. This distinct clinical entity can occur with both posterior stabilized and cruciate retaining designs and is characterized by painful subluxation of the tibia on the femur. This situation is often seen in patients in whom the total knee prosthesis is well aligned axially and well fixed.²⁹ Manifestations of flexion instability differ depending on the cause of the problem and the implant design.

A large flexion gap may result from inadequate filling with the implant, often due to under-resection of the distal femur in an attempt to avoid elevating the joint line. In this scenario, the surgeon will typically preferentially balance the resulting tight extension gap at the expense of a loose flexion gap. With trials in place, the tibia should have less than 5 mm of anteroposterior translation when the knee is tested intraoperatively at 90 degrees of flexion with the patella reduced.³⁰ The “figure-of-four” position, as described earlier, can be used to establish that the knee will not dislocate.

Dislocation of a posterior stabilized total knee arthroplasty is rare but dramatic, with a reported incidence between 0% and 0.5%.^9,^14,²² If the flexion gap is large enough, the cam mechanism may jump over the post, resulting in posterior dislocation of the tibia on the femur. Dislocation typically occurs with marked knee flexion plus a valgus or varus stress (such as occurs when crossing the legs to put on socks or shoes).⁸ At-risk patients include those who had correction of a large valgus deformity, particularly if they quickly regained knee flexion postoperatively.³⁴ First-time dislocations should be treated with closed reduction, a trial of bracing, and avoidance of the activity that induced the dislocation, although severe laxity will often lead to repeated dislocations. Mobile-bearing designs may require open reduction.¹³ Recurrent dislocation may require revision with thicker polyethylene (if the extension space permits). A constrained condylar implant may be helpful but will not prevent repeat dislocation in the setting of continued gap imbalance (Fig. 129-5). The “Figure 4” maneuver can be used to assess correction of the instability.