a case of transverse patella fracture

A Case of Transverse Patella Fracture

Clinical Vignette

A thin 78-year-old male with a past medical history of T2DM, HTN, BPH, and renal cell carcinoma presents to clinic after a ground level fall to the right knee. He reports a 7/10 pain overlying the patella. The following xray is obtained:
Knee XR Transverse Patella Fracture Lateral View
Lateral radiograph showing transverse fracture of the patella.
Which of the following is a reassuring exam finding with this injury?

A) Intact extensor mechanism
B) Negative anterior drawer test
C) Absent joint effusion
D) Increased knee ROM

Introduction

A transverse patellar fracture occurs when the patella fractures into two parts in the transverse plane, dividing the bone into an upper and lower portion. Patellar fractures constitute about 1% of all skeletal fractures. Roughly 50-80% are the transverse type that are likely to disrupt the extensor mechanism of the knee[1]Heusinkveld MH, den Hamer A, Traa WA, Oomen PJ, Maffulli N. Treatment of transverse patellar fractures: A comparison between metallic and non-metallic implants. British Medical Bulletin. … Continue reading. It is commonly caused by direct trauma to the patella or a fall. 80% of patella fractures occur in the middle to lower third of the patella with up to two-thirds of them displace[2]Jarraya M, Diaz LE, Arndt WF, Roemer FW, Guermazi A. Imaging of patellar fractures. Insights Imaging. 2017 Feb;8(1):49-57. doi: 10.1007/s13244-016-0535-0. Epub 2016 Nov 30. PMID: 27905071; PMCID: … Continue reading.

A transverse fracture of the patella may result in swelling, inability to bend or straighten the knee, inability to bear weight, stand, or walk. Treatment may be operative or conservative depending on physical exam and radiographic findings.

Anatomy

Knee Extensor Mechanism
Illustration of the extensor mechanism from the anterior view.[3]Image courtesy of intechopen.com, “Surgical Approaches for Total Knee Arthroplasty”
The patella is the largest sesamoid bone in the body and is located anterior to the knee joint within two tendons, the quadricep tendon superiorly that provides an attachment point for the quadriceps femoris muscle, and the patellar tendon inferiorly that attaches to the tibial tuberosity. It assists in knee extension and acts as a lever arm to the quadriceps tendon as it exerts on the femur by increasing the angle at which it acts. Without the patella, more force would be required to achieve knee extension.

The patella is variable in its shape, however it is typically ovoid. It assists with the extensor mechanism along with the quadriceps tendon, medial and lateral retinacula, and patellar tendon. The extensor mechanism is responsible for extending the knee and maintaining an erect, extended position. The patella serves three main functions: provides protection of the knee joint, lengthens the lever arm of the quadriceps femoris to allow for better knee extension, increases the area of contact between the patellar tendon and the femur[4]White TD, Black MT, Folkens PA. Chapter 12 – Leg: Femur, Patella, Tibia, and Fibula. In: Human Osteology. San Diego, CA: Academic Press; 2011..

Diagnosis

X-ray imaging is most often used in the diagnosis of a transverse patellar fracture with a sensitivity and specificity approaching 100%[5]Avci M, Kozaci N. Comparison of X-Ray Imaging and Computed Tomography Scan in the Evaluation of Knee Trauma. Medicina (Kaunas). 2019 Sep 23;55(10):623. doi: 10.3390/medicina55100623. PMID: 31547588; … Continue reading; however it has been shown that CT imaging provides more accurate evaluation to assess if there is comminution in other areas of the patella. Ultrasound has also been used as a first line imaging modality to diagnose patellar fractures with a sensitivity and specificity of 93.3%% and 94.8% respectively[6]Kilic TY, Yesilaras M, Atilla OD, Turgut A. The accuracy of point-of-care ultrasound as a diagnostic tool for patella fractures. The American Journal of Emergency Medicine. 2016;34(8):1576-1578. … Continue reading. MRI may be used if there is concern for underlying occult fracture.

It is important to differentiate between a fractured patella and a bipartite patella, a congenital condition of the patella in which there is an unfused accessory ossification center (a two-part patella). The superolateral accessory ossification center of the patella is usually present by age 12 and may persist into adulthood. A bipartite patella is 9 times more common in males than females and occurs in ~2% of the population and bilaterally in ~43% of cases[7]Rich J, Powers RH, Dean DE. Forensic Medicine of the Lower Extremity: Human Identification and Trauma Analysis of the Thigh, Leg, and Foot. Scholars Portal.

Physical Exam

Physical exam may reveal tenderness, redness, and swelling overlying the patella. The edges of the fracture may also be felt through the skin, especially if the fracture is displaced. It is important to identify any defects in the quad tendon, patella or patella tendon. Note if the patella is high or low riding. Hemarthrosis is typically present. Range of motion should be assessed, and the extensor mechanism should be evaluated to see if it is intact. The examiner should consider a structural exam depending on the mechanism and patients tolerance.

Management

Transverse patellar fractures may be displaced or nondisplaced. If the fracture is nondisplaced or minimally displaced and the knee can be maintained in an extended position (intact extensor mechanism), non-operative treatment with bracing/immobilization is possible. The goal is to keep the leg in an extended position to allow for approximation of the bones. When the knee is flexed, there is a superior and inferior tension in the patella from the quadriceps and patellar tendons respectively that will prevent healing. Commonly used immobilizers include the T-scope brace or Zimmer knee splint.
Surgical intervention is required for fractures that interrupt the extensor mechanism or demonstrate > 2 to 3 mm step-off and >1 to 4 mm of displacement[8]Melvin SJ, Mehta S. Patellar fractures in adults. American Academy of Orthopaedic Surgeon. 2011;19(4):198-207. doi:10.5435/00124635-201104000-00004.

Case Conclusion

Knee XR Transverse patella fracture 12 weeks follow up
Follow up at xray at 12 weeks shows well healed fracture without further displacement.
Upon presentation, this patient’s physical exam was reassuring. On inspection, he had mild swelling of the right knee with tenderness to palpation overlying the patella. He had full flexion of his knee. Extension was limited to 10 degrees, however he had an intact extensor mechanism. X-ray imaging showed a transverse fracture of the patella with approximately 3mm inferior displacement. He was placed into a T-scope brace and repeat imaging 12 weeks later demonstrated the following imaging with a well healed transverse intra-articular fracture of the patella and maintained extension.

Author: Dr Kareem Shahin

– More Knee Pain from Sports Medicine Review: https://www.sportsmedreview.com/by-joint/knee/

Vignette Answer

As discussed in the case presentation, the most important physical exam component in the setting of patella fractures is evaluation of the extensor mechanism. A negative drawer test is reassuring if you are able to evaluate this. Hemarthrosis is common following a patella fracture and range of motion is often decreased due to pain and swelling.

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