Patella Fractures: Approach to Treatment

Damayea I. Hargett, MD; Brent R. Sanderson, DO; Milton T.M. Little, MD

Disclosures

J Am Acad Orthop Surg. 2021;29(6):244-253. 

In This Article

Patella Biomechanics

The patella serves as an articulating fulcrum to increase the moment arm of the extensor mechanism. In addition, it notably improves the efficiency of the quadriceps muscle by elevating the extensor mechanisms from the axis of rotation of the knee and increases the torque generated. It also aids in reducing frictional wear that would otherwise deteriorate the extensor mechanism tendon. When the knee is fully flexed, the patella is a link between the quadriceps and patellar ligament. Daily activities generate patellofemoral compressive forces of 3.3 times body weight while climbing stairs and 7.6 times body weight while squatting.[3] The patella engages the femur from 45° flexion to full extension, and this displaces the extensor mechanism from the mechanical axis of the knee, increasing torque generation to allow for terminal extension.[1,4]

Mechanism of Injury

Patellar fractures may result from either direct or indirect forces but usually involve a combination of the two forces. A direct blow to the patella usually results from a ground level fall or dashboard injury from a motor vehicle collision. This mechanism of injury typically results in a comminuted/stellate fracture pattern with articular injury. Indirect trauma typically occurs when the mechanical properties of bone are overcome by eccentric loading forces. The patella fails under tension by rapid knee flexion against a contracted quadriceps. These indirect injuries present as a transverse fracture pattern with larger displacement, retinacular injury, and less articular injury/impaction.[5]

Clinical History and Assessment

A thorough history and physical examination must be done for patients presenting with anterior knee pain. High-energy dashboard knee injuries should be evaluated for associated femoral neck fractures, posterior wall acetabular fractures, and hip dislocations. Athletes presenting with indirect mechanisms or twisting mechanisms should also be assessed carefully for associated injuries. It has been reported that 95% of patients with acute lateral patella dislocations have articular cartilage injuries to the patellofemoral joint.[6]

A physical examination may reveal swelling, palpable defects, lacerations, or abrasions. Detection of traumatic arthrotomy of the knee can be accomplished with the detection of intra-articular air on CT as recent literature has demonstrated improved sensitivity and specificity compared with a saline load test.[7] Competence of the extensor mechanism should be assessed using an active straight leg raise test. To avoid a false-positive secondary to pain, a knee aspiration and intra-articular injection of local anesthetic may be required.

Imaging

AP, lateral, and two oblique knee radiographs should be obtained for patella fractures. This four radiograph combination has been shown to notably improve sensitivity for fracture detection in comparison with conservative two radiographic views (AP, and lateral).[8] Four views including the use of a flexed lateral view may provide additional information to guide surgical decision-making and help determine retinacular integrity.

Advanced imaging may be considered for comminuted, occult, or stress fractures. CT scan has been shown to affect surgical management plans in 49% of patients and change fracture classification in 66% of patients assessed.[9] MRI is highly sensitive for the detection of occult fracture, cartilage damage, and subchondral fracture and provides additional information regarding the integrity of the extensor mechanism; however, this is not indicated for acute displaced patella fractures.

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