A patellar tendon rupture involves a complete tear of the tendon that runs from the patella's inferior pole to the tibial tubercle. It is typically seen in males in their third or fourth decade of life. It tends to result from an overall weakened tendon placed under high tensile forces. These tendon ruptures are best classified into acute versus chronic tears, depending on the time from rupture. This type of injury requires prompt diagnosis and surgical repair as the patellar tendon is a part of the extensor mechanism. The extensor mechanism of the knee is crucial to the function of the lower extremity, including ambulation. It is solely responsible for extending and straightening the knee as well as resisting knee flexion – a crucial aspect of standing with a flexed knee and, therefore, ambulation. Without a properly functioning knee extensor mechanism, the patient is severely limited functionally. Surgical intervention depends on timing and location of the rupture. Acute ruptures are amenable to primary repair whereas chronic ruptures often require tendon reconstruction.
The knee extensor mechanism is composed of the quadriceps muscle, quadriceps tendon, medial and lateral patellar retinaculum, patella, patellar tendon, and tibial tubercle.
The quadriceps muscle is composed of four separate muscles with different origins but a common insertion point on the patella through the quadriceps tendon.
Quadriceps muscles with origins
The medial and lateral patellar retinaculum are on their respective sides of the patella and are continuous with the vastus fascia to the tibia and the patella. They are minor patellar stabilizers and, if intact, can provide knee extension and straight leg raising despite a patellar or quadriceps tendon rupture.
The patella is a sesamoid bone. It functions to increase the moment arm from the knee joint axis, thereby increasing the mechanical advantage and quadriceps pull in extension. The patella begins to engage the trochlea at 20 degrees of flexion and is fully engaged by 40 degrees of flexion. Joint reaction forces in the patellofemoral joint can be up to three times the body weight with stair climbing and seven times the body weight with deep bending.
The patellar tendon, by definition, is a ligament as it connects bone (patella) to bone (tibial tubercle). The patellar tendon is approximately 30 mm wide by 50 mm long, with a thickness of 5 to 7 mm. The origin on the inferior pole of the patella is juxtaposed on the articular cartilage on the deep side and becomes confluent with the periosteum of the patella anteriorly. The tibial insertion is narrower and invests the entirety of the tibial tubercle, connecting the quadriceps muscles to the lower leg.
Tendon rupture usually is the result of underlying tendinosis or a weakened tendon. 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 predispose an individual to tendon rupture as well.
Disruption of the knee extensor mechanism can occur at different locations within the extensor mechanism chain. The three most common areas of disruption include the patella, the quadriceps tendon, and the patellar tendon. Patella fractures are much more common than tendon ruptures by more than double. Quadriceps tendon rupture tends to be more common than patellar tendon ruptures, especially in individuals over 40 years of age. In the United States, quadriceps tendon ruptures tend to affect 1.3% of the population each year, whereas patellar tendon ruptures tend to affect less than 0.5% of the population each year. 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.
Patellar tendon rupture occurs as a result of a tensile overload on the extensor mechanism. This is usually the result of long-standing chronic tendon degeneration. The usual circumstance is the quadriceps muscle suddenly contracts with the knee in a flexed position. 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 amount of force on the patellar tendon is seen when the knee is flexed more than 60 degrees. As a result, the majority of patellar tendon ruptures occur with the knee in a flexed position. Patellar tendon rupture can occur at three distinct locations with a proximal avulsion of the tendon, with or without bone from the inferior pole of the patella being the most common. The strain at the tendon-inferior pole patella interface is three to four times higher than at the midsubstance of the tendon. The other two possible locations for rupture include the midsubstance of the tendon and an avulsion of the patellar tendon from the tibial tubercle.
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, it is essential to obtain a detailed history of their symptoms. The history of the present illness should include the onset of their symptoms, specific location of their pain, duration of the pain and symptoms, characteristics of the pain, alleviating and aggravating factors, any radiation of pain, and the severity of their symptoms. Patients may report pre-existing pain at the level of the patella or patellar tendon which may be indicative of an underlying tendinosis. Furthermore, a thorough history may reveal an underlying risk factor or predisposition to a tendon rupture.
There are differing opinions on the definition of an acute versus chronic tendon rupture; however, in general, chronic ruptures are those presenting six 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 often will be associated with a large hemarthrosis and surrounding ecchymosis. Inspect and evaluate the patellar height and compare the affected and unaffected side. A patellar tendon rupture will likely be associated with an elevation of the patellar height compared to the uninjured side.
Palpation of the bony and soft tissue structures is an essential part of any knee exam. The palpatory exam can be broken down into the medial, midline, and lateral structures of the knee.
Palpatory Exam - Areas of Focus 
Medial aspect of the knee
Lateral aspect of the knee
Patients with a patellar tendon rupture will have a palpable defect below the inferior pole of the patella. They will also have 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 will have decreased ROM of the knee, not only due to pain but also due to disruption of the extensor mechanism. There will be a loss of active knee extension, which is the key physical exam finding. If the patellar tendon is the only portion of the extensor mechanism ruptured and the retinaculum is intact, the active extension may be possible, but there will be an extensor lag of a few degrees. Patients will be unable to perform active straight leg raise and also will be unable to maintain a passively extended knee.
Care must be taken to avoid missing a diagnosis of a patellar tendon rupture or any disruption of the extensor mechanism, 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 will not be able to perform a straight leg raise despite adequate local anesthesia. On the other hand, a patient with a painful effusion secondary to a different issue will be able to perform a straight leg raise.
Recommended radiographic evaluation includes anteroposterior and lateral views of the affected knee. In a complete rupture, radiographs may reveal patella alta, or a superiorly displaced patella. The Insall-Salvati ratio is a quick method to determine patella alta or baja (inferiorly displaced patella) on the lateral knee radiograph. It is defined as the ratio between the length of the patellar tendon and the length of the patella. 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. Radiographs also may reveal avulsion fractures or other concomitant knee injuries. An MRI of the knee 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 is helpful in determining 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 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 in those instances where 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 immobilization with the knee in full extension with a progressive weight-bearing exercise program.
Surgical treatment includes primary tendon repair or tendon reconstruction. Primary repair is indicated in complete patellar tendon ruptures and in cases in which the tendon ends can be approximated. The location of the tear will dictate the type of repair used. An end-to-end repair will generally be used when the tendon tear is midsubstance. A transosseous tendon repair, with bone tunnels drilled through the patella, will generally be used for a proximal avulsion. A suture anchor tendon repair will generally be used for a distal avulsion. Tendon reconstruction is indicated in severely disrupted or degenerative patellar tendons or in cases in which 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.
Autograft and Allograft Tissue Options
Overall, there are good to excellent outcomes reported with patellar tendon ruptures that undergo surgical repair promptly. Poor outcomes, complications, and failures are typically associated with missed or delayed diagnosis, delayed treatment, or technical errors during surgery.
The post-operative rehabilitation protocol will vary based on the surgeon, but the following is a general guideline.
Weeks 0 through 2
Weeks 2 through 6
Weeks 6 through 12
Weeks 12 through 16
Weeks 16 and Longer
Return to Sport Criteria
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