Extensor mechanism (EM) disruption is an uncommon complication following total knee arthroplasty. Disruption can occur at the level of the patellar tendon, the patella with a concomitant fracture, or the quadriceps tendon. The overall incidence of extensor mechanism disruption has been reported between 1.1 and 6.6 % [1]. Risk factors include multiply operated knees, systemic conditions such as renal disease, diabetes, systemic steroid use, rheumatoid arthritis, and obesity [2, 3]. Iatrogenic injury at the time of surgery is unfortunately a common cause. Iatrogenic injury stems from failure to protect the extensor mechanism, especially in a patient with a stiff knee where exposure may be difficult and common particularly in revision surgery.

Quadriceps tendon rupture is the least common site of disruption. Dobbs et al. identified 24 of 23,800 TKA patients (0.1 %) from the Mayo Clinic with a partial or complete quadriceps tendon rupture [4]. Several risk factors exist. Aggressive resection of the patella can compromise the insertion of the quadriceps. Vascular injury may be a risk factor as well. In the previous referenced study, all three patients with quadriceps tendon rupture had undergone a lateral retinacular release [4]. This may cause injury to the superior lateral genicular artery and place the tendon at heightened risk for rupture.

Patellar fractures after TKA usually do not interrupt the extensor mechanism. Vertical fractures can typically be treated without surgery unless there is component loosening. More recently, the incidence of all patellar fractures has ranged from 0.3 to 5.4 % [5]. A study from Mayo clinic identified 85 fractures after 12,464 TKAs (0.68 %) of which only 12 were associated with extensor mechanism rupture [6]. Sinha et al. discussed three major risk factors that statistically increased the risk of patella fracture: resurfacing or over-resection of the patella, implant malalignment, or compromise of the patellar blood supply via lateral retinacular release [5]. All efforts should be made to prevent these risk factors.

Patellar tendon rupture or avulsion has been reported in as many as 2.5 % of patients with TKA [2]. Risk factors include difficult exposure in the setting of revision surgery, previous high tibial osteotomy, previous patellar realignment surgery, or systemic disease. Several techniques to gain exposure have been recommended to protect the patellar tendon during surgery including resection of intra-articular scar, quadriceps snip, and tibial tubercle osteotomy.

In this case presentation, the disruption of the extensor mechanism occurred at the level of the patella following a traumatic fall. Primary repair of the patella and/or extensor mechanism in the setting of total knee arthroplasty often leads to suboptimal results and is associated with high rates of failure as discussed in the outcomes section below. In this case, attempts to repair the patella fracture with suture anchors led to suboptimal fixation and failure requiring extensor mechanism reconstruction.

When assessing a patient with possible extensor mechanism disruption, it is important to perform a thorough history and physical exam. While often times the extensor deficiency is obvious, there are several critical issues that should be addressed that have a key role in the etiology and ultimately the outcome of the extensor mechanism reconstruction. These elements include the chronicity of the injury, previous surgeries, or complications from previous surgery, particularly if they involved the extensor mechanism. The patient’s current health and functional status is also important. Every patient should be questioned and evaluated about signs and symptoms of infection.

The physical exam should focus on the function of the extensor mechanism as well as the status of the soft tissue envelope. Inability to perform active knee extension or inability to do a straight leg raise indicates dysfunction of the extensor mechanism. A palpable defect in the patellar tendon or quadriceps may be felt on exam, along with the presence of a “high riding” patella if the patella tendon is disrupted. Often the patient may present with a large effusion in the knee that is subcutaneous from extravasation of the joint fluid from the disrupted extensor mechanism. In addition to infection, instability or component mal-rotation may be the cause for extensor mechanism disruption and must be addressed prior to the reconstruction. Failure to correct the errors at the time of surgery will lead to high failure rates of the reconstruction and the surgeon must be prepared to revise all necessary components at the time of extensor mechanism reconstruction as needed.

Plain radiographs are often all that is needed to assist in the diagnosis. The lateral radiograph may show patella alta, or a high riding patella as in this case (Fig. 10.1). Both the femoral and tibial components should be evaluated for loosening, osteolysis, and malposition.

CT scan and MRI are useful adjuncts in the diagnosis of patellar tendon ruptures if it is not readily evident on physical exam and plain radiographs. Recent advances in metal artifact reduction sequences have allowed better characterization of the remaining host tissue to assist in surgical decision making. CT scan can be used to determine femoral or tibial mal-rotation that may require revision at the time of surgery.

The surgeon also needs to assess the patient’s candidacy for a relatively lengthy rehabilitation process that will require strict postoperative protocols. Contraindications to extensor mechanism reconstruction would include active periprosthetic infection and medical comorbidities precluding surgical intervention as well as a patient who is unwilling or unable to comply with postoperative protocols.