Extensor Mechanism Deficiency
Matthew P. Abdel
Arlen D. Hanssen
Extensor mechanism deficiency following total knee arthroplasty (TKA) can occur owing to disruption of the patellar tendon or the quadriceps tendon or catastrophic patellar fracture. Each situation requires a slightly different anatomic reconstructive approach.
Principles of the reconstruction depend on whether the extensor discontinuity is acute, subacute, or chronic. If chronic, the quadriceps mechanism retracts proximally and scars to the adjacent soft tissues and femur.
The rotational position and status of prosthesis fixation should be determined either preoperatively or immediately after surgical exposure as loose or malrotated implants require concomitant revision.
Some patients with chronic extensor deficiency develop varying degrees of global instability that may be amenable to simple upsizing of tibial insert only.
If there is concomitant deep periprosthetic infection, a 2-stage approach is recommended. In most cases, owing to the extensor mechanism disruption, a nonarticulating spacer will be required as articulating spacers are prone to tibiofemoral dislocation.
Extensor mechanism reconstruction has historically been done with whole extensor or Achilles tendon allografts. However, our preferred technique is to use synthetic mesh for reconstruction. The rationale for using mesh is that multiple sutures passed through friable host tissue are stabilized and more secure when those multiple sutures also pass through the synthetic mesh graft.
Sterile Instruments and Implants
Routine revision TKA instruments and implants
Specific implants and tibial inserts for partial revision cases
Cancellous bone screws
5-0 and #1 nonabsorbable suture
10″ × 14″ sheet of knitted monofilament polypropylene mesh (Marlex mesh; C.R. Bard, Inc., Murray Hill, NJ, USA) (Figure 73.1)
Important note: this surgical mesh should be the variety that allows soft-tissue ingrowth and not one of the newer variety meshes that prevent tissue ingrowth.
Use of a sterile tourniquet that allows for an extremely proximal skin incision is recommended.
Utilize the most recent and reasonable skin incision and anticipate considerable proximal extensile exposure to identify and mobilize the quadriceps, particularly in chronic cases.
Determine whether existing implants are well fixed and/or well positioned.
Obtain prosthesis records for indwelling implants and arrange for these specific implants in the event that partial revision might be performed with the extensor mechanism reconstruction.
If prior reconstructive attempts with allograft have been performed, it is helpful to obtain operative records to review that surgeon’s description of technique to facilitate complete removal of allograft tissue.
Bone, Implant, and Soft Tissue Techniques
Depending on the location of the extensor mechanism disruption, the choice of how to perform the arthrotomy is critical.
Patellar tendon disruption. In these patients, the distal tissue is often thin and redundant. Midline division of this tissue to create equal medial and lateral tissue flaps up to the inferior pole of the patella is important for eventual wound closure and distal coverage of the mesh.
If the patella is absent, simply extending the arthrotomy in the midline is performed; otherwise, a standard medial parapatellar arthrotomy is performed.
Quadriceps tendon disruption. In these patients the medial and lateral halves of the quadriceps are usually retracted proximally and to their respective sides. The arthrotomy in these circumstances should be performed adjacent to the medial edge of the vastus lateralis to maximize the amount of tissue that remains attached to the vastus medialis muscle. This is very helpful for eventual distal and lateral translation of the vastus medialis at final closure.
If the patella is present, distal extension of a parapatellar arthrotomy is performed; otherwise, the arthrotomy is carried in the midline down to the tibial tubercle.
Important note: Do not resect any redundant native healthy tissue, particularly distally, as often this tissue is required for arthrotomy closure and coverage of the mesh.
Determination is made as to whether the tibial and femoral components require revision or whether a tibial liner exchange will confer adequate joint stability.
The 10″ × 14″ sheet of mesh is first prepared by folding it to a width of 15 mm that is 8-ply thick (Figure 73.1). Thereafter, it is unitized with a single #5 nonabsorbable suture placed in a running locked fashion along the free margin.