An 8-year-old girl is brought to the orthopedic clinic 3 weeks after treatment of a supracondylar fracture of the humerus. She had undergone closed reduction and percutaneous pinning under general anesthesia. On examination, she has limited elbow extension (extension lag of 15°) with normal flexion. Grip strength is intact. Plain radiographs show good fracture alignment with intact pins. What is the most likely cause of her limited extension?

Explanation

## Extension Lag After Supracondylar Fracture Fixation ### Clinical Presentation Analysis - **Extension lag of 15°** = inability to actively extend the elbow fully, though passive extension is normal - **Intact flexion** = biceps and brachialis function preserved - **Intact grip strength** = hand intrinsics and forearm muscles intact - **Good fracture alignment** = mechanical reduction is adequate ### Key Point: **Extension lag with preserved flexion and grip strength = brachialis dysfunction.** The brachialis is the primary elbow extensor (innervated by musculocutaneous nerve), and its injury or entrapment is the most common cause of post-reduction extension lag. ### Mechanism of Brachialis Injury in Supracondylar Fractures 1. Anterior displacement of the proximal fragment stretches the brachialis 2. During reduction, the brachialis may be **trapped between fracture fragments** 3. Direct trauma or entrapment during pinning can damage muscle fibers 4. Muscle injury → weakness in elbow extension ### Differential Diagnosis Table | Nerve/Muscle | Motor Deficit | Sensory Loss | Grip Strength | Extension | Flexion | |---|---|---|---|---|---| | **Brachialis injury** | Extension lag | None | Intact | **Weak** | **Normal** | | AIN injury | Thumb IP flex loss | None | **Weak** | Normal | Normal | | Radial nerve | Wrist/finger extension loss | Dorsal thumb/index | Intact | Normal | Normal | | Musculocutaneous nerve | Flexion weakness | Lateral forearm | Intact | Normal | **Weak** | ### High-Yield: **Extension lag ≠ nerve injury if grip strength is intact.** This finding is **muscular**, not neurological. Brachialis entrapment is the classic complication of supracondylar fracture reduction. ### Clinical Pearl: Brachialis injury is often **self-limiting**. Most patients recover full extension within 3–6 months with physiotherapy. Surgical exploration is rarely needed unless there is no improvement after 6 months. ### Management - Physiotherapy and active-assisted range of motion - Serial examination to assess recovery - Surgical exploration only if no improvement by 6 months [cite:Canale & Beaty Operative Pediatric Orthopedics 2e Ch 8]