Introduction

The patella serves a critical role in the function of the extensor mechanism by increasing its moment arm. Functioning as a sliding lever, it translates the quadriceps and patellar tendons anterior to the trochlea and the knee’s axis of rotation, which generates compressive forces across the patellofemoral joint. These forces can exceed three times the body weight while climbing stairs and as much as seven times the body weight while squatting. Patellar fracture is the most common cause of disruption of the extensor mechanism and is approximately six times as common as either quadriceps or patellar tendon rupture. These fractures account for 0.5 to 1.5% of skeletal injuries, with the majority resulting either from falls from heights or from traffic accidents.

I. Preoperative

1. 
   

   History and physical examination

   
   
   
   
   1. 
      

      The majority of these fractures occur in patients 20 to 50 years of age, with a 2:1 male-to-female ratio. They may occur either as a result of direct or indirect forces or as a combination of the two.

      
      
   2. 
      

      Examination may reveal a large, tense hemarthrosis. Widely displaced fractures in which the retinacula have been disrupted may result in a less tense effusion as the capsule is disrupted.

      
      
   3. 
      

      Failure to perform a straight leg raise or to actively extend the knee. Nondisplaced or minimally displaced fractures may spare the retinacula, preserving the patient’s ability to perform a straight leg raise.

      
      
   4. 
      

      Palpable defect.

      
      
   5. 
      

      Open fractures occur in 6 to 30% of patellar fractures. The majority of open fractures (up to 75%) are Gustilo–Anderson type II.

   
   
2. 
   

   Anatomy

   
   
   
   
   1. 
      

      Osteology:

      
      
      
      
      1. 
         

         The patella is the largest sesamoid bone in the body. Its articular surface is divided longitudinally by a central ridge that separates the medial and lateral facets (▶ Fig. 38.1 ). The odd facet is a third, smaller facet, and is located more medially.

         

         
         
      2. 
         

         The proximal three-quarters of the patella is covered by cartilage, which can be up to 1-cm thick. The distal quarter of the patella is nonarticular and devoid of cartilage.

         
         
      3. 
         

         Bipartite patella results from failure of fusion of the accessory patellar ossification center and occurs in approximately 2 to 8% of the population and are bilateral in 50% of cases. The most common location is superolateral. Unlike a fracture, a bipartite fragment will appear well corticated on radiographs and lack edema on MRI (▶ Fig. 38.2 ).

         

      
      
   2. 
      

      Neurovascular anatomy:

      
      
      
      
      1. 
         

         The blood supply is formed by an anastomotic ring surrounding the patella (▶ Fig. 38.3 ). This is formed by the confluence of six arteries: the supreme genicular artery, medial and lateral superior genicular arteries, medial and lateral inferior genicular arteries, and the anterior tibia recurrent artery.

         

         
         
      2. 
         

         The supreme genicular artery, also known as the descending genicular artery, arises from the superficial femoral artery. The anterior tibial artery gives rise to the anterior tibial recurrent artery. The remaining four vessels arise from the popliteal artery. The inferior portion of the ring is formed by the transverse infrapatellar branch, which runs within the infrapatellar fat pad, deep to the patellar tendon.

         
         
      3. 
         

         The intraosseous blood supply comes from mid-patellar vessels, which enter on the anterior surface in the middle third of the bone to supply the proximal two-thirds and from polar vessels, which come from the transverse infrapatellar branch to the inferior third of the patella.

         
         
      4. 
         

         The dominant blood supply enters retrograde through the inferior pole. As a result of this retrograde blood flow, displaced transverse fractures may place proximal fragments at risk of avascular necrosis (AVN).

         
         
      5. 
         

         The infrapatellar branch of the saphenous nerve provides sensation to the anterior knee and anterolateral leg. It branches from the saphenous nerve distal to the adductor canal and courses from superomedial to inferolateral crossing the patellar tendon approximately 3 cm inferior to the patella.

      
      
   3. 
      

      Ligamentous and tendinous insertions:

      
      
      
      
      1. 
         

         Medial patellofemoral ligament (MPFL): It originates on the adductor tubercle and inserts on the superomedial portion of the patella. It resists lateral displacement of the patella (▶ Fig. 38.4 ).

         

         
         
      2. 
         

         Lateral patellofemoral ligament (LPFL): It originates on the proximal lateral epicondyle and inserts on the superolateral portion of the patella. It resists medial displacement of the patella (▶ Fig. 38.4 ).

         
         
      3. 
         

         The vastus intermedius originates on the anterior surface of the femur and inserts directly on the superior pole of the patella and forms the deep layer of the aponeurosis.

         
         
      4. 
         

         The remainder of the quadriceps (rectus femoris, vastus lateralis, vastus medialis, vastus medialis obliquus (VMO), and the vastus lateralis obliquus) terminate in an aponeurosis that merges into the anterior third of the joint capsule and is superficial to the vastus intermedius.

         
         
      5. 
         

         The aponeurosis courses over and is adherent to the ventral surface of the patella. It continues to course distally and is contiguous with the superficial portion of the patellar tendon.

         
         
      6. 
         

         The VMO is a dynamic stabilizer and prevents lateral subluxation of the patella.

         
         
      7. 
         

         The patellar tendon extends from the inferior pole of the patella to the tibial tubercle.

   
   
3. 
   

   Imaging

   
   
   
   
   1. 
      

      Plain radiographs:

      
      
      
      
      1. 
         

         Anteroposterior, lateral, and skyline or merchant views (▶ Fig. 38.5 ).

         

         
         
      2. 
         

         Plain radiographs underestimate the degree of comminution and articular incongruity.

      
      
   2. 
      

      Advanced imaging:

      
      
      
      
      1. 
         

         CT scan can be used to further delineate the extent of comminution. The addition of a CT scan has been shown to lead to changes in management plans in nearly 50% of cases.

         
         
      2. 
         

         MRI is not routinely used but may be beneficial for occult or osteochondral fractures.

   
   
4. 
   

   Classification

   
   
   
   
   1. 
      

      Descriptive classification:

      
      
      
      
      1. 
         

         Nondisplaced, transverse, vertical, stellate, distal pole, and osteochondral (▶ Fig. 38.6 ).

         

         
         
      2. 
         

         Sixty-five percent of fractures are nondisplaced.

         
         
      3. 
         

         Transverse fractures account for 50 to 80% of patellar fractures. The majority of these are in the middle to lower third of the patella.

         
         
      4. 
         

         Vertical fractures account for up to 20% of patellar fractures. Vertical fractures are typically not displaced and do not compromise the extensor mechanism.

         
         
      5. 
         

         Stellate pattern fractures may be seen in 30 to 35% of cases.

         
         
      6. 
         

         Osteochondral fractures occur as the result of a patellar dislocation.

         
         
      7. 
         

         Frontal plane fractures have been reported but are exceedingly rare.

      
      
   2. 
      

      AO/OTA classification:

      
      
      
      
      1. 
         

         The patella is labeled 34.

         
         
      2. 
         

         Fracture types include the following: A, extra-articular; B, partial articular (vertical); and C, complete articular (nonvertical).

         
         
      3. 
         

         Fractures are then further subclassified based on location (medial vs. lateral and proximal vs. distal) and the degree of comminution.

         
         
      4. 
         

         This classification is not commonly used in practice and is primarily reserved for research.