Patellar Tendon Rupture

Etiology

Chronic inflammation, such as patellar tendonitis, leads to a weakened tendon and can increase the likelihood of tendon rupture. Certain medical conditions can lead to an overall weakened tendon and can also predispose an individual to tendon rupture.

Risk Factors

• Systemic lupus erythematosus
• Rheumatoid arthritis
• Chronic renal disease
• Diabetes mellitus
• Renal dialysis
• Chronic corticosteroid use
• Fluoroquinolone antibiotics
• Corticosteroid injections
• Patellar tendinopathy
• Previous injury
• Overuse injury
• Patellar degeneration[5][2][6][7] 

Epidemiology

Disruption of the knee extensor mechanism can occur at different locations within the extensor mechanism chain. The 3 most common areas of disruption include the patella, the quadriceps tendon, and the patellar tendon.[8] Patella fractures are much more common than tendon ruptures by more than double.[9] Quadriceps tendon rupture tends to be more common than patellar tendon ruptures, especially in individuals over 40.[10] In the United States, quadriceps tendon ruptures tend to affect 1.3% of the population yearly, whereas patellar tendon ruptures tend to affect less than 0.5% of the population yearly. Males are more commonly affected than females. The thought behind this is that males are physically stronger and more susceptible to rupture of the extensor mechanism. Additionally, females have greater ligamentous laxity, and hormonal changes due to the menstrual cycle may be protective.

Pathophysiology

Patellar tendon rupture occurs due to a tensile overload on the extensor mechanism. This is usually the result of long-standing chronic tendon degeneration.[11] The usual circumstance is that the quadriceps muscle suddenly contracts with the knee flexibly. This can be seen in cases where an individual was running up a set of stairs, landing from a jump, or suddenly stopping to change directions when running. The greatest force on the patellar tendon is seen when the knee is flexed more than 60 degrees. As a result, most patellar tendon ruptures occur with the knee in a flexed position. Patellar tendon rupture can occur at 3 distinct locations, with a proximal avulsion of the tendon, with or without bone from the inferior pole of the patella, being the most common.[6] The strain at the tendon-inferior pole patella interface is 3 to 4 times higher than at the mid-substance of the tendon. The other 2 possible rupture locations include the tendon's mid-substance and an avulsion of the patellar tendon from the tibial tubercle.

History and Physical

Patients with an acute patellar tendon tear present to their healthcare provider with the complaint of infrapatellar knee pain, swelling, difficulty with weight-bearing, and difficulty straightening the leg. They may report an audible “pop” or the sensation of their knee giving way during an event with a sudden quadriceps contraction with the knee in a flexed position, such as with jumping sports or missing a step on the stairs. As with any presenting complaint, obtaining a detailed history of their symptoms is essential. The history of the present illness should include the onset of the symptoms, the specific location of the pain, the duration of the pain and symptoms, characteristics of the pain, alleviating and aggravating factors, any radiation of pain, and the severity of the symptoms. Patients may report pre-existing pain at the patella or patellar tendon level, which may indicate underlying tendinosis. Furthermore, a thorough history may reveal an underlying risk factor or predisposition to a tendon rupture.[12] There are differing opinions on the definition of acute versus chronic tendon rupture, but the general thought is that chronic ruptures present 6 weeks after injury. Physical examination of the knee should begin with an inspection. Be sure to evaluate the surrounding skin for any signs of direct trauma. Look for any associated swelling or knee effusion. Patellar tendon ruptures are often associated with a large hemarthrosis and surrounding ecchymosis. Inspect and evaluate the patellar height and compare the affected and unaffected sides. A patellar tendon rupture is likely associated with an elevation of the patellar height compared to the uninjured side. Palpation of the bony and soft tissue structures is essential to any knee exam. The palpatory exam can be broken down into the knee's medial, midline, and lateral structures.

Palpatory Exam - Areas of Focus

Medial Aspect of the Knee

• Vastus medialis obliquis
• Superomedial pole patella
• The medial facet of the patella
• Origin of the medial collateral ligament (MCL)
• Midsubstance of the MCL
• Broad insertion of the MCL
• Medial joint line
• Medial meniscus
• Pes anserine tendons and bursa

Midline Knee

• Quadricep tendon
• Suprapatellar pouch
• Superior pole patella
• Patellar mobility
• Prepatellar bursa
• Patellar tendon
• Tibial tubercle 

Lateral Aspect of the Knee

• Iliotibial band
• Lateral facet patella
• Lateral collateral ligament (LCL)
• Lateral joint line
• Lateral meniscus
• Gerdy’s tubercle[13] 

Patients with a patellar tendon rupture have a palpable defect below the inferior pole of the patella and localized tenderness about the infrapatellar aspect of the knee. Range of motion (ROM) testing and muscle strength testing are essential aspects of the knee exam, especially in the setting of a suspected patellar tendon rupture. Patients with an acute patellar tendon rupture have decreased ROM of the knee due to pain and disruption of the extensor mechanism. There is a loss of active knee extension, which is the key finding of the physical exam. Suppose the patellar tendon is the only portion of the extensor mechanism ruptured, and the retinaculum is intact. The active extension may be possible in that case, but an extensor lag of a few degrees. Patients are unable to perform active straight leg raises and also unable to maintain a passively extended knee. Care is necessary to avoid missing a diagnosis of a patellar tendon rupture or any extensor mechanism disruption, as delayed diagnosis and treatment affect the outcome. If needed, aspiration of a painful knee effusion followed by an injection of lidocaine may be performed to aid in clinical diagnosis. A patient with an extensor mechanism disruption cannot perform a straight leg raise despite adequate local anesthesia. On the other hand, a patient with a painful effusion secondary to a different issue can perform a straight leg raise.

Evaluation

Recommended radiographic evaluation includes anteroposterior and lateral views of the affected knee. In a complete rupture, radiographs may reveal patella alta (superiorly displaced patella). The Insall-Salvati ratio quickly determines patella alta or baja (inferiorly displaced patella) on the lateral knee radiograph. It is defined as the ratio between the patellar tendon's length and the patella's length. Ideally, this is measured on a lateral radiograph with the knee flexed to 30 degrees. A normal ratio is between 0.8 and 1.2, with patella alta having a ratio greater than 1.2 and patella baja having a ratio of less than 0.8.[4]

Radiographs also may reveal avulsion fractures or other concomitant knee injuries. A knee MRI is an appropriate diagnostic study if a patellar tendon rupture is suspected. It is the most sensitive imaging modality and can differentiate partial from complete tendon rupture. It helps determine the exact location of the rupture, the presence of any tendon degeneration, the position of the patella, and any concomitant intraarticular knee lesions. Ultrasound also may be used as an adjunctive study in the suspected case of acute or chronic patellar tendon rupture. It is effective in detecting and localizing tendon disruption. It is much less expensive than obtaining an MRI and can be more convenient depending on the availability of an ultrasound machine and an experienced user.

Treatment / Management

Treatment for complete patellar tendon ruptures involves surgical repair, as disability from a deficient knee extensor mechanism is high. Although not considered to be a surgical emergency, prompt surgical management of acute patella tendon ruptures is recommended to prevent the need for reconstruction. Nonsurgical treatment for patellar tendon ruptures is only indicated when the tendon tear is partial, and there is an intact knee extensor mechanism. The other instance in which nonsurgical management should be considered would be for patients who are not surgical candidates due to medical comorbidities. Non-surgical treatment involves immobilizing the knee in full extension with a progressive weight-bearing exercise program.

Surgical treatment includes primary tendon repair or tendon reconstruction.[8] Primary repair is indicated in complete patellar tendon ruptures, where the tendon ends can be approximated.[14] The location of the tear dictates the type of repair used. An end-to-end repair is generally used when the tendon tear is mid-substance. A transosseous tendon repair, with bone tunnels drilled through the patella, can generally be used for a proximal avulsion. A suture anchor tendon repair is generally used for a distal avulsion. Tendon reconstruction is indicated in severely disrupted or degenerative patellar tendons or in cases where primary repair cannot be performed. Tendon excursion, adhesion, and degeneration increase as the time from initial injury to surgical repair increases. This can convert a simple primary tendon repair to a more complicated patellar tendon reconstruction. This makes it important to treat patellar tendon ruptures with a sense of urgency. The patellar tendon can be reconstructed from autograft or allograft tissue. There are multiple surgical techniques described for patellar tendon reconstruction.[15][16](A1)

Autograft and Allograft Tissue Options

• Semitendinosus
• Gracilis
• Ipsilateral or contralateral central quadriceps tendon-patellar bone
• Achilles tendon with a bone block

Differential Diagnosis

Differential diagnosis for patellar tendon rupture include:

• Quadriceps tendon rupture
• Patella fracture
• Tibial tubercle avulsion fracture

Prognosis

Overall, promptly repaired patellar tendon ruptures have good to excellent outcomes. Poor outcomes, complications, and failures are typically associated with missed or delayed diagnosis, treatment, or technical errors during surgery.

Complications

Complications that can manifest with patellar tendon rupture are as follows:

• Re-rupture
• Residual extensor mechanism weakness
• Residual extensor lag or inability to fully extend the knee
• Knee stiffness
• Quadriceps atrophy
• Infection