The Extensor Mechanism in Total Knee Replacement

The extensor mechanism plays a vital role in total knee replacement (TKR) in the primary and the revision setting. Currently, the most common complication requiring revision arthroplasty after successful TKR involves some aspect of the extensor mechanism from patellar dislocation to extensor mechanism disruption, including patella fracture and abnormal patellar wear causing symptomatic synovitis. ³ This session will discuss the general aspects of the approach to the extensor mechanism in primary and revision arthroplasty.

Patella tendon ruptures are extremely rare in primary TKR, but are more common in revision arthroplasty, particularly in patients with restricted range of flexion. Restricted ROM frequently is responsible for intraoperative avulsion of the patellar tendon because of the inability to flex the knee beyond 90° with associated scarring in the lateral gutter producing excessive tension on the patellar tendon during the surgery. ¹ Avoidance can be accomplished with either a proximal soft tissue release or a tibial tubercle osteotomy in individuals in whom lateral retinaculum release and soft tissue releases do not allow adequate excursion to avoid rupture. Unless one faces either a patella alta or patella baja, this is accomplished most commonly with a proximal release, avoiding the bony healing required in a tibial tubercle osteotomy. Because the patella tendon is ruptured during the surgery, repair can be accomplished by direct suture with or without augmentation, but results from this surgery generally have been less than excellent. ⁴ In some cases, the rupture is not obvious during the time of the surgery, but can occur with either overly aggressive postoperative rehabilitation, falls with acute flexion, and/or manipulation. All of these may result in a patellar tendon rupture. Other preventive mechanisms include either screw and washer through the tibial tubercle, or use of an AO clamp, with one portion of the clamp positioned beneath the MCL and beyond the midsagittal plane of the tibia, and the second portion placed on the lateral aspect of the tibial tubercle so that the pressure on the patellar tendon is minimized during the surgical procedure.

Fractures of the patella are most common after the surgical procedure because of decreased overall thickness of the patella. This is most common in patellas that have been decreased to less than 10 mm when the risk of a fracture significantly increases, particularly with acute flexion. These fractures may occur at any time after surgery and range from simple avulsion fracture of either the distal or proximal poles to significant midsubstance fractures with or without loss of continuity of the patella implant composite. Depending on the size, location, and disability, surgical techniques for fracture fixation with or without reimplantation of the patella implant will be described.

Patella subluxation and dislocation are infrequent complications of TKR. In fact, these fall into three different groupings ²: (1) lack of adequate external rotation of the tibial component; (2) lack of adequate external rotation of the femoral component; and (3) lack of medialization of the patella implant on the native bone and lack of adequate retinacular release.

Almost all implant manufacturers include a deeper and more conforming trochlea to provide better capture of the patella and, because the materials have improved gradually with respect to strength, this can be accomplished without compromise of the overall strength of the implant. Many manufactures provide a left and a right femoral implant to allow better tracking and decreased shear and compression forces on the patella component. In addition, the surgical approach, whether it is a midvastus versus a rectus approach, has had a significant effect on the number of lateral retinacular releases necessary during primary arthroplasty.

Controversy remains regarding the necessity of resurfacing all patellas during TKR. The decision-making process as to which patellas potentially should not be resurfaced also remains in question. The literature would suggest that overall resurfacing is a better option; however, there are isolated series in which specific patient populations may be candidates for nonresurfacing techniques. Certainly in revision arthroplasty in which the patella segment or native patella bone already is compromised, there are occasions when adequate thickness does not allow for a repeat resurfacing procedure requiring either allowing the remaining patella remain in its native state versus some of the new augmentations that now are under trial.

Quadriceps rupture is a rare initial complication of TKR, but because of attrition of the rectus it may, in fact, provide a significant late complication rate. If diagnosed initially, massive tears usually can be repaired with overall successful results. However, if found late or if secondary to significant devascularity of the rectus tendon, the ultimate outcome is in jeopardy.

This symposium will discuss factors affecting the decision-making process associated with the treatment of extensor mechanism abnormalities of TKR and surgical techniques of initial and repeat repair.