Tuesday, July 30, 2019

Extensor Mechanism Injuries

The extensor mechanism comprises of the quadriceps muscles and tendon, medial and lateral retinacula, patella, patellar tendon, and tibial tubercle. Tendons of the extensor mechanism are extremely resistant to tensile loads and do not rupture under normal physiologic conditions, even with significant degrees of stress. Injury generally happens due to sudden vigorous contraction of the muscle with the knee in a flexed position, laceration, or a direct impact. Disruption may occur at any level from the quadriceps muscle to the insertion on the tibial tubercle. 






Rupture of the quadriceps tendon usually occurs at or just proximal to the patellar inser- tion. Occasionally the rupture may extend into the vastus intermedius tendon or transversely into the retinaculum. Most patellar tendon ruptures occur at the site of origin on the inferior pole of the patella.


Quadriceps tendon rupture > 40 years 
Patellar tendon rupture < 40 years


Risk Factors for Extensor Mechanism Injuries - Chronic systemic conditions, including rheumatoid arthritis, gout, systemic lupus erythematosus, hyperparathyroidism, and iatrogenic immunosuppression in organ transplant recipients, use of steroids/fluoroquinolones. 


Patients with delayed diagnosis of patellar tendon rupture may experience significant retraction of the patella proximally and subsequent development of quadriceps contractures or adhesions. 


Clinical Features. Clinical evaluation can elicit the correct diagnosis in most cases of complete disruption. Classical signs are:
1. Acute onset of pain, swelling, and ecchymoses over the anterior aspect of the knee and a palpable defect in the patella, quadriceps tendon, or patella tendon
2. Loss or limitation of ability for active leg extension - extension lag usually is seen when the last 10 degrees of extension is performed haltingly or with difficulty)
3. High- riding patella (patella alta) with patellar tendon rupture and superior retraction
4. Low-riding patella (patella baja) with quadriceps tendon rupture and inferior retraction. 

Partial disruptions may not show these clinical signs and may require MRI for confirmation.


Diagnostic Imaging
AP and Lateral X Rays Knee

  • Obliteration of the quadriceps or patella tendon, a poorly defined suprapatellar or infrapatellar soft tissue mass (represents proximal or distal retraction of the torn tendon), soft tissue calcific densities (represent avulsed bone fragments of the patella or tibial tubercle), or a displaced patella.
  • Patella alta may be sought on the lateral radiograph using a ratio of patellar length to patellar tendon length (the Insall-Salvati ratio). The Insall-Salvati ratio (TL/PL) is considered normal between 0.8 and 1.2. Patella baja: <0.8, patella alta: >1.2. 
Ultrasound has low sensitivity and specificity in diagnosing acute quadriceps and patellar tendon ruptures. MRI shows the entire extensor mechanism and is the best imaging modality for diagnosing pathology in this system, even in the acute phase. MRI usually is reserved for patients with possible incomplete disruption or for those with a complication of intra-articular derangements. 



Management
Early Repair - within 2 to 6 weeks of the initial injury. If the tear is only partial, immobilization with the knee in full extension for 4 to 6 weeks is the treatment of choice. Surgical intervention is required for reattachment of complete tendon ruptures, and repair should be performed as soon as possible. After primary repair, the knee is immobilized in full extension with a long leg cast until healing is complete. Gradually progressive active and passive range-of-motion exercises are indicated for optimal results.


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     Lakshay Chanana
     
     ST4 Trainee
     Royal Infirmary of Edinburgh
     Department of Emergency Medicine
     Edinburgh
     Scotland

     @EMDidactic


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