Extensor mechanism knee injuries are a common clinical entity seen by physicians in a variety of clinical settings. The extensor mechanism is critical for normal gait, activities of daily living, and sports. Injuries to this process can be divided into traumatic and non-traumatic mechanisms.
It is critical to understand the anatomy of knee extension. From proximal to distal, the primary extensors anatomy of the knee includes the quadriceps muscle, the patella, and the patellar tendon. The quadriceps muscle group includes the rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis. The rectus femoris originates from the anterior inferior iliac spine, the vastus lateralis from the lateral greater trochanter, the vastus medialis from the femoral neck, and the vastus intermedius from the femoral body. These muscles converge to form the quadriceps muscle tendon, which attaches to the superior pole of the patella. Synchronous contraction of these muscles results in lower leg extension. The patella fits within the trochlear groove of the femur and acts as an anchor for the quadriceps and patellar tendons. The patellar tendon originates at the inferior pole of the patella and inserts into the tibial tuberosity.
The retinacula of the extensor mechanism are composed of connective fibers from the quadriceps muscle group and are divided into medial and lateral portions. They include the patellofemoral, vastus medialis, medial collateral, patellotibial, and medial patellomeniscal ligaments. These ligaments provide stability to the osseous components of the knee.
The soft tissue structures of the knee include the quadriceps, prefemoral, and infrapatellar fat pads. Lastly, the suprapatellar, pretibial, and prepatellar bursae provide a cushion against friction and assist in movement. The blood supply to the structures of the knee is provided by the lateral and inferior genicular arteries, which are branches of the femoral artery. The nerve supply is provided by branches of the tibial, femoral, obturator, and common peroneal nerves.
Risk factors for this disease can be divided into two categories: weakening of the collagen and local injuries to the area.
Risk factors causing weakening of collagen include systemic illnesses such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), chronic kidney disease, diabetes mellitus, hyperparathyroidism, and iatrogenic, including chronic steroid and fluoroquinolone use.  Connective tissue disorders, including the generic title of ‘hypermobility syndrome,’ Ehlers Danlos syndrome, and Marfan syndrome, can also predispose patients to extensor injuries. Patients on hemodialysis are most likely to have tendon degeneration resulting in rupture.
Local injuries include patellar instability, patellar tendinopathy, or degeneration. Patellar instability can be congenital or due to anatomical variants. The most common causes of instability include patellar tilt, patella alta, trochlear hypoplasia, or vastus medialis weakness. Tendinopathies and degeneration can result from chronic overuse or repeated traumatic injuries.
Quadriceps tendon ruptures are rare, and patellar tendon ruptures even more so. They have an incidence of 1.37 and 0.68, respectively, per 100,000 people. Quadriceps tendon ruptures are more common in male patients above the age of 40. Patellar tendon ruptures require significant force and are more common in athletes under the age of 40.
Patellar fractures comprise 1% of all skeletal injuries and tend to be associated with direct trauma to the patella, such as in a car accident or a fall. They are most common in patients aged 20 to 50 years and are twice as common in men. Closed fractures are most common, but open fractures occur in 7% of cases. Patellar fractures are twice as common as patellar tendon ruptures.
Patellar dislocations comprise 3% of all knee injuries, and 70% of these injuries occur in patients younger than 20. Dancers, other athletes, and those in military service are more prone to dislocations. Dislocations can result from direct trauma or indirect trauma to a knee with inherent patellar instability. The incidence of dislocation decreases with age. Patellar subluxation is a partial dislocation that self-resolves and is more common than a complete dislocation.
Osgood-Schlatter disease is an apophysitis of the tibial tubercle that is most common in young teens, especially males. Onset typically coincides with increasing physical activity or a recent growth spurt. Females will present anywhere from 8 to 13 years of age, while males can present from 10 to 15 years of age due to a later age of growth plate maturation.
The most common cause of knee pain in young athletes is idiopathic chondromalacia patellae (ICP), often referred to by its more generic name of patellofemoral syndrome, or patellofemoral pain syndrome. ICP is an overuse or overload injury, and is more prevalent in women (at a rate of 2 to 10 times that of men), runners, and obese patients. Idiopathic knee pain affects close to 30% of all adolescents.
Traumatic knee extensor injuries usually involve quadriceps contraction against a flexed knee, such as when a patient is attempting to regain balance after a fall, jumping, or missing a step on the stairs. Significant injuries may also occur in patients with comorbidities after a simple fall. Patients may hear a popping sound or feel a tearing sensation. They will likely complain of knee pain and swelling with an inability to bear weight.
Quadriceps tears typically occur where the tendon inserts into the patella and may cause an inferiorly displaced patella (patella baja). Patellar tendon tears have three common patterns of injury. The most common is the avulsion of the tendon from the inferior pole of the patella. Other injuries include a mid-tendon tear and a distal tendon avulsion from the tibial tubercle. These injuries typically result in a superiorly displaced patella (patella alta).
Patellar fractures are typically caused by trauma directly to the patella, such as a fall, or indirectly through a forced contraction. Patellar fractures can be vertical, marginal, transverse, or comminuted. Vertical fractures may not cause disruption of the extensor mechanism.
The patella can be dislocated in any direction, including intra-articular, but the most common is a lateral dislocation. In traumatic dislocations, the mechanism is typically a twisting motion of the knee or direct force to an externally rotated foot. Frequently, the patella will have subluxed and spontaneously reduced before the patient is seen by a physician. Dislocations have a high rate of recurrence and are complicated by patellofemoral instability and patellofemoral arthritis. Some patients with chronic patellar dislocations are able to self-reduce their injuries. Dislocation inherently causes a tear to the medial patellofemoral ligament (MPFL). The MPFL, along with the lateral patellofemoral ligament (LPFL), provides medial and lateral (respectively) counter-resistance to the patella.
Osgood Schlatter is caused by a combination of fast bone growth and repetitive stress on the patellar tendon insertion at the tibial tubercle. The growth of bone is faster than the ability of the patellar tendon to stretch, causing tension over the tibial apophysis. Repetitive stress can cause avulsion and inflammation of the apophysis, termed apophysitis. Constant stress on the inflamed apophysis may eventually lead to an avulsion fracture of the physis (epiphysiodesis) if left untreated (*mechanism and predisposing factors for proximal tibial epiphysiolysis in adolescents during sports activities). Typical activities that worsen pain include jumping, walking upstairs, and running. Symptoms of Osgood Schlatter typically are self-limited and resolve once the apophysis fuses with the tibial epiphysis in early adulthood.
The mechanism of idiopathic chondromalacia patellae is complex and multifactorial; it includes components of degenerative changes, history of participation in sports, obesity, patellar maltracking, and muscular imbalances which apply increased stress on the anterior knee.
The physical exam for a suspected knee injury should include inspection, palpation, range of motion, neurovascular examination, and strength. The range of motion can be divided into active and passive components. An inspection may show deformity, or hemarthrosis of the knee, ecchymoses, abrasions, or open fractures. Depending on the injury, a joint effusion may or may not be present. Patients may also have an asymmetric patella (patella baja or patella alta) when compared to the unaffected knee, due to a tear of the supporting tendon.
The most specific physical exam finding for a complete quadriceps or patellar tendon tear is the loss of the extensor mechanism, i.e., the inability to raise the leg against gravity while extended. Patients with partial tears may demonstrate painful, impaired extension of the knee.
Patients with patellar fractures may have point tenderness over the patella, knee effusion, limited range of motion, limited or nonexistent leg extension, or patella alta or baja.
Patients with patellar dislocations usually present with a laterally dislocated patella, with a limited range of motion and knee extension. They may give a history of prior dislocations or patellar instability, or of a direct injury. The exam may be limited due to an effusion, which indicates a potentially more complicated injury pattern, such as an osteochondral injury. It is important to evaluate distal pulses to ensure there is no vascular compromise.
In the case of non-traumatic knee pain such as Osgood-Schlatter and idiopathic chondromalacia patellae, patients may report a long history of anterior knee pain, worse with activities and knee extension. The pain will improve with rest. Patients with ICP may give a history of typical symptoms such as anterior knee pain worse with standing up after prolonged sitting (“movie-goer’s knee”).
Patients with Osgood Schlatter will typically have point tenderness over the tibial tubercle at the insertion of the patellar tendon, and symptoms can be reproduced by knee flexion or resisted knee extension. This is a clinical diagnosis, and imaging is not usually necessary to confirm the diagnosis.
The initial imaging modality of choice is radiographs in virtually all cases of knee pain, whether traumatic or atraumatic. Standard views include anterior-posterior (AP), lateral, and oblique. A sunrise view can be obtained to evaluate the patella better. Fractures can be distinguished from a bipartite patella, an anatomical variant, by the orientation of the fracture lines and irregular fractures pieces. In the case of tendon ruptures, plain films may show avulsion fractures where the ligament has torn, a knee effusion, patella alta, or baja, or they may be completely normal. Patellar dislocations appear on plain film most commonly as a laterally displaced patella with an associated effusion.
Point-of-care ultrasound can be used as an initial imaging modality in the right clinical context, especially if a tendon injury is suspected and should be used as an adjunct to radiographs when available. Partial tears may show incomplete disruption of the tendon fibers along with a hypoechoic area near the site of injury. Complete tears will demonstrate an obvious hypoechoic area with tendon visible proximally and distally depending on the degree of retraction. Color Doppler may show increased vascularity as a result of the injury.
In select cases, computed tomography (CT) may be the imaging modality of choice. In patients with extensor mechanism injuries, CT is most useful when a fracture is suspected, as this can help delineate the fracture pattern and allow for surgical planning. In most cases, CT is not required. MRI also has a role in evaluating extensor mechanism injuries. In cases of suspected tendon rupture, the surgical team can use this information to evaluate for partial vs. complete tears and plan their surgical approach. If the patient is effused, an intra-articular injury has also occurred, and this can help identify any pathology in the joint.
Treatment depends on the preservation of the extensor mechanism. Any injury causing impaired knee extension usually requires surgical repair and orthopedic consultation. The prognosis is favorable if the injury is diagnosed and treated early.
Treatment for partial extensor injuries may depend on the patient. Young, active patients would benefit from early surgical repair to regain function. Older patients with significant comorbidities may benefit from a trial of nonoperative management, which requires placing the patient in a knee immobilizer with instructions for a progressive range of motion and weight-bearing plan. Whether an operative or nonoperative course is trialed, patients should expect to return to full activity after 6 months of extensive rehabilitation.
Treatment for patellar fractures again depends on the preservation of the extensor mechanism, as well as the type of fracture. Conservative treatment is appropriate for stable, non-displaced fractures with a preserved extensor mechanism. To test this, the fracture must remain stable when the knee is flexed up to 60 degrees. Comminuted fractures may be treated nonoperatively if they fall into this category. Vertically oriented fractures are more likely to be stable and heal while horizontal or transversely oriented fractures are at higher risk of further displacement, non-union, and loss of extension. The patient should be placed in a knee immobilizer with a limitation of knee flexion. The degree of flexion and weight-bearing is increased in a stepwise fashion. Surgical repair is appropriate for unstable fractures, or fractures that impair the extensor mechanism.
For patients with patellar dislocations, if the patella is still dislocated on examination, it may be reduced based on clinical diagnosis alone without imaging. However, if there is any concern for associated bony injuries, or the patient is elderly with a first-time dislocation, a plain film should be considered before reduction. If the patella is unable to be reduced, orthopedic surgery should be consulted. If the injury is successfully reduced on the initial evaluation, the patient can be placed in a knee immobilizer with instructions to follow up with orthopedic surgery or sports medicine in 1 to 2 weeks. In most cases, conservative management is trialed first. Patients with a medially dislocated patella may require surgery for reduction. Patients with chronic dislocation may benefit from operative repairs, such as MPFL reconstruction or graft, to aid in patellar stability.
As Osgood Schlatter disease and ICP are overuse tendinopathies, patients should be advised to rest from strenuous physical activity. Other recommendations include activity modification, ice packs, and NSAIDs. Physical therapy focuses on the strengthening of quadriceps, especially VMO, as well as core, glute, and hamstrings. Eccentric exercises are preferred for tendinopathies. Jumping biomechanics may need to be corrected in athletes if there is any genu varum or valgum.
The differential for knee pain is wide and can be categorized into traumatic and non-traumatic injuries.
In general, patients report improved functional outcomes and satisfactory results when knee extensor mechanism injuries are treated early. Poor outcomes are associated with delayed treatment, diagnosis, or technical surgical errors.
Complications depend on the extent of damage and delay of diagnosis or treatment. These complications include:
Impaired knee extension
Any injury causing complete tears or a significant reduction in the extensor mechanism of the knee requires orthopedic consultation for possible surgical repair. Orthopedic colleagues should also be consulted in cases of concomitant fractures.
Patients should be counseled on the favorable prognosis of most causes of knee extensor injuries. They should be advised to adhere to the rehabilitation guidelines in regards to weight-bearing and physical activity.
Knee extensor injuries relate to the damage of structures that support the extensor mechanism of the knee joint. When damaged, these injuries may cause debilitating pain and swelling, with loss of ability to straight leg raise and limited knee extension. Some patients may describe a 'pop' sensation in the knee joint before the injury. Complications of these injuries can lead to loss of motion, knee stiffness, and osteoarthritis. It is, therefore, important to treat these injuries early. Depending on the severity, they may warrant surgical management. Patients will benefit from an interprofessional healthcare team, including primary physicians, surgeons, sports medicine, nurses, and physical therapists to return to normal activity. Rehabilitation and orthopedic nurses provide education and monitor patient progress and keep the team apprised of patient response to treatment. [Level 5]
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