The patella, colloquially known as the kneecap, is the largest sesamoid bone in the body. It is formed within the quadriceps femoris muscle. It has a role in protecting the knee joint, protecting the quadriceps tendon from friction forces and has a significant role in maximizing the biomechanics of knee extension. It articulates with the femur.
The patella is triangular in shape with its apex pointed inferiorly to which the patella ligament is attached. The base (superior border of triangle) is broad and serves as an attachment point for quadriceps femoris tendon. The lateral border has a tendinous insertion from vastus intermedius. The medial border has a tendinous insertion from vastus medialis. Note all four of the quadriceps muscles contribute to the quadriceps femoris tendon thus it is often referred to as the quadriceps tendon.
The anterior surface is convex with rough longitudinal striae and is covered by a thin layer of the quadriceps tendon which is then continuous with the superficial fibers of the patella ligament. Between this and the skin is the prepatellar bursa.
The posterior surface articulates with the femoral trochlea of the distal femur and has a medial and lateral facet that meet in the midline to form a raised vertical ridge. The lateral facet is the larger of the two. Below the articular surface area is a rough convex area not covered by hyaline cartilage to which the patellar ligament attaches.
The patella articulates with the femoral trochlea and is considered to be part of the knee joint.
The patella has both an extra and intraosseous blood supply. A plexus of blood vessels, sometimes known as the anastomotic patella ring, makes up the extraosseous blood supply. This is contributed to by the following; the supreme genicular, medial superior genicular, lateral supe-rior genicular, medial inferior genicular, lateral inferior gen-icular, and anterior tibial recurrent arteries(1, 4). The superior contributions and anastomosis run anterior to the quadriceps tendon, while the inferior contributions and anastomosis runs posterior to the patella ligament through the fat pad.
The intraosseus blood supply is made up of two major systems. One is the midpatellar vessels which enter the patella on the middle third of the anterior surface. The second polar vessels enter the distal pole of the patella between the attachement of the ligamentum patella and the articular surface(1). Due to the distribution of intraosseous blood flow there is a risk of avascular necrosis (usually of the upper half) following fracture(4).
No nerve roots have been found within the knee joint itself or the patella(1, 5). L2 through to L5 supply the anterior cutaneous innervation of the region. Anteriomedial innervation is made up of the femoral, obturator and saphenous nerves. Anterolateral innervation is made up of the later femoral and lateral sural cutaneous nerves.
Patella ligament: The patella ligament is also at times referred to as the patella tendon, however as it connect bone to bone ligament is perhaps the better terminology. It originates from the distal apex and margins of the patella and inserts into the tibial tuberosity. Deep to this ligament is the infrapatellar fat pad and deep infrapatellar bursa.
Medial patellofemoral ligament (MPFL): Is a band of retinacular tissue connecting the femoral medial epicondyle to the medial edge of the patella(6). It has been stated that the MPFL provides 50-80% of the restraining force to lateral patella dislocation(7).
Lateral patellofemoral ligament (LPFL): Is a thickening of the knee joint capsule and the anatomical findings of this ligament vary(8). The ligament is anchored at the lateral femoral epicondyle and attaches to the lateral patella(8, 9).
At 9 weeks gestation the region of the future patella becomes chondrified forming a hyaline cartilaginous mass that segments the formerly continuous tendinous insertion of the quadriceps and distally fibrocartilagenous transition into the patellar ligament occurs(1). By 23 weeks gestation the patella has lateral facet predominance and increases in relative size until 6 months gestation, after which its growth is relative(2). Ossification begins from a primary ossification center in a centrifugal manner; typically this commences around the age of five years and is completed around puberty(1, 3).
Accounts for 1% of all fractures usually occurring between the age of 20 and 50 years being more common in men than women(10). The mechanism of injury is direct trauma, sudden forceful muscle contraction and iatrogenic (i.e. during knee arthroplasty)(11).
Plain film radiography can be useful in diagnosis, an anteroposterior, lateral and tangential (Merchant) views should be obtained. Many opt for computer tomography for added detail, including greater articular step assessment, and pre-operative planning. MRI is at times used for further assessment of soft tissues and in cases of dislocation(12).
There are multiple fracture types including transverse, vertical, stellate (comminuted), apical, marginal and osteochondral. The fracture type and patient factors such as function will help guide management.
As with all fractures one can contemplate a conservative or surgical management pathway. Factors favoring conservative management are a lack of significant displacement or articular incongruity, closed fracture, intact extensor mechanism, when the extensor mechanism forces are perpendicular to the direction of fracture displacement (vertical fractures) and when patients have a high co-morbid burden or low functional need (i.e. non ambulatory)(10, 11). Conservative management involves immobilization of the leg in near full extension for around six weeks using a cast or leg brace until evidence of healing on radiographs, the patient can often partially weight bear with crutches(10). Note that controlled and timely physiotherapy is key to a satisfactory outcome.
There are multiple surgical options for patellar fixation with very limited randomized controlled trials comparing the techniques available(10). Aims of operative treatment are to preserve extensor function, vascularization, and restore articular congruency. Options include modified tension-band wiring, lag screw fixation, cerclage, cannulated lag screw with tension band, partial patellectomy, and total patellectomy(12). Tension band wiring is the most widely used technique, total patellectomy should be a last resort due to resulting poor function(11, 12).
Patellar dislocation accounts for 2-3% of all knee injuries and is the second most common cause of traumatic haemarthrosis (first in children), it usually occurs between the age of 10 and 20 years(12, 13). The mechanism of injury can be traumatic or relatively atraumatic on a background of ligamentous laxity. Classically the patella is displaced laterally out of the trochlear groove. In the nearly all lateral displacements the MPFL is disrupted(12, 14).
Many dislocations spontaneously reduce themselves (often on knee extension) and thus may reduced on attendance to the emergency department. Patient history alongside examination is therefore important, it is worth remembering soft tissue swelling and patient apprehension will likely make examination challenging. Imaging can consist of plain film radiographs and/or computer tomography (CT) to investigate bony injury and assess for predisposing bony factors. Magnetic resonance imaging (MRI) can also be used to visualize the soft tissues including the MPFL.
Management of non-complicated primary patella dislocation is a topic of debate. Initially the aim in all patella dislocations (regardless if first time or recurrent) is to provide symptomatic relief that will consist of; relative rest, light compression, elevation, ice and analgesia(13). It then becomes unclear on the advantages between conservative and surgical management. Many will advocate non-surgical management with rehabilitation following first dislocation, however some advocate repair of the MPFL(13). However there is minimal quality evidence available on the topic and a recent Cochrane review was unable to confirm a significant difference between either approach due to the lack of quality evidence(15).
For recurrent dislocations surgery becomes an increasingly attractive option. The procedures available consist of MPFL reconstruction, trochleoplasty and trochlear osteotomy. However there is very poor evidence comparing surgical intervention to rehabilitation in these cases(13, 15).
Acute patella tendon rupture
Acute patella tendon rupture tends to occur in those under the age of 40 years when the extensor mechanism is overloaded(16, 17). Rupture is relatively rare and in the healthy adult a force of 17.5 times the patients body weight is required(18). However in those with previous patella tendon pathology, steroid injections, hyperparathyroidism, rheumatoid arthritis, immunosuppression, systemic lupus erythematosus the risk increases(17).
The patient will often present with severe sudden onset pain, swelling, inability to weight bear and weakness of the extensor mechanism(16). On examination one will find swelling and may find a high riding patella (Patella Alta) and/or weakened extensor ability. However even in complete ruptures some retinacular fibers may be intact facilitating extension(16). Initial imaging should be plain film radiographs demonstrating a high riding patella, some may use ultrasonography which can in dynamic studies be useful in differentiating between complete and partial rupture(16). MRI is also a useful investigation.
In complete rupture swift surgical repair is necessary to maximize outcome, however if partial rupture non-surgical management with immobilization in extension can be undertaken(16). Physiotherapy input is required in all cases to aid rehabilitation.
Patellofemoral osteoarthritis (PFOA) is not uncommon and is radiographically evident in isolation in 24% of those over 50 years and is more common in females(19, 20). The causes are similar to osteoarthritis in other areas of the body including trauma, obesity and inflammatory processes. However in the case of PFOA patellar misalignment and tilt contributes(21).
The patient will most often complain of anterior knee pain aggravated my going up or down stairs, squatting and raising from sitting. Stiffening and pseudolocking may also be a presenting complaint(21). Physical examination may reveal antalgic gait, mild effusion, crepitus and tenderness. There may also be muscle wasting.
Plain film radiography is the initial imaging modality of choice; AP, lateral and Merchant view should be obtained. Computer tomography can add further detail and is especially useful if maltracking is suspected, it also gives further information on whether osteoarthritis is present in other areas of the knee joint(21).
Conservative treatment consists of muscle strengthening, exercise, weight loss, activity modification, oral analgesia and intra-articular corticosteroid injection. However in some cases this will not result in adequate symptom relief and surgery may be considered.
There are many techniques with potential use in PFOA. The procedure can be joint preserving (lateral release, tibial tuberosity osteotomy) or joint replacing (patellofemoral replacement, total knee arthroplasty)(21, 22). The principle of correct patient selection for the correct procedure is key in the case of PFOA intervention.
Development: Cartilaginous patella in utero, ossification begins around 5 years of age and completed around puberty.
Blood supply: Extraosseous patellar plexus. Intraosseous supply from polar vessels and mid patella vessels.
Nerve supply: No nerve root within the patella itself. L2-5 anterior cutaneous supply.
The patella is the largest sesamoid bone in the human body and has a vital role in maximizing the biomechanics of the knee joint. Clinically pathology of the patella can be a cause of considerable pain and morbidity in both the young and the older patient. A good working anatomical knowledge can aid the clinician in their diagnosis and management of patella pathology.
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