What is the management of a hyperextended knee?

Management of Hyperextended Knee

Immediate management of acute knee hyperextension injury requires initial immobilization with a knee brace set at 5-10 degrees of flexion (not neutral) to prevent extension deficit, combined with early quadriceps strengthening exercises once the acute injury phase resolves. 1, 2

Acute Phase Management (First 3 Weeks)

Bracing Strategy

• Set the knee brace at -5 to -10 degrees of extension (slight flexion), NOT at 0 degrees (neutral). Standard post-injury bandaging and braces set at 0 degrees fail to maintain full extension and can lead to extension deficits 2
• Radiologic studies demonstrate that braces set at -5 degrees achieve proper knee positioning, while -10 degrees ensures all knees reach straight or slight hyperextension 2
• This bracing approach prevents the common complication of extension deficit that occurs in 55% of patients when neutral braces are used (versus only 9% with -5 degree braces) 2
• Functional knee braces with hinge-post-shell design provide superior tibial-displacement control and should be considered for ligamentous instability 1

Initial Assessment Priorities

• Evaluate for bicruciate ligament injury. Pure hyperextension mechanisms generate simultaneous ACL and PCL ruptures in experimental models, with peak failure occurring at approximately 34 degrees of hyperextension 3
• The posteromedial capsule and posterior oblique ligament (21.7% of restraining force), posterolateral capsule and fabellofibular ligament (17.1%), and cruciate ligaments (ACL 13%, PCL 12.9%) are the primary structures injured 4
• High anterior compartment compressive forces during hyperextension cause anterior tibial subluxation contributing to ACL tension, while excessive extension simultaneously damages the PCL 3

Rehabilitation Phase (Weeks 3-12)

Exercise Prescription

• Begin daily quadriceps strengthening with sustained isometric exercises for both legs, regardless of unilateral injury. 1, 5
• Include proximal hip girdle muscle strengthening, as these muscles provide critical knee stability 1
• Add aerobic conditioning and range of motion/stretching exercises as adjunctive therapy 1
• Follow the "small amounts often" principle—link exercises to daily activities (before morning shower or meals) rather than treating them as separate events 1
• Start within the patient's capability and build intensity over several months 1

Activity Modification

• Consider walking aids (cane on contralateral side) to reduce pain and improve participation during recovery 1, 6
• Use appropriate comfortable footwear to reduce adverse mechanical factors 1

Pain Management Algorithm

First-Line Therapy

• Acetaminophen up to 4,000 mg/day is the preferred initial oral analgesic. 1, 5, 7

Second-Line Options

• Topical NSAIDs have clinical efficacy with superior safety profiles and should be tried before oral NSAIDs 1, 5, 6
• Topical capsaicin provides additional pain relief options 1

Third-Line Therapy

• Oral NSAIDs only if unresponsive to acetaminophen, used at lowest effective dose for shortest duration 1, 5, 7
• Intra-articular corticosteroid injections for acute pain flares, especially with effusion, providing relief up to 3 months 6, 7

Special Considerations

Congenital Hyperextension (Arthrogryposis)

• Very early treatment immediately after birth is critical 8
• Initial traction and mobilization followed by serial casting greatly improves range of motion 8
• Surgical lengthening of extensor apparatus may be required for persistent hyperextension deformities 8

Post-Surgical Hyperextension

• Custom orthoses can correct hyperextension complications following total knee replacement 9
• The goal is restoration of functional ambulation and independence in activities of daily living 9

Critical Pitfalls to Avoid

• Never use a brace set at 0 degrees (neutral) in the acute phase—this leads to extension deficits in over half of patients 2
• Do not assume isolated ACL injury with hyperextension mechanism—bicruciate injuries occur simultaneously and require comprehensive evaluation 3
• Avoid neglecting the posterior capsular structures (posteromedial and posterolateral), which provide 54.7% of the total restraining moment against hyperextension 4
• Do not delay quadriceps strengthening once acute phase resolves—muscle rehabilitation is more important than prolonged bracing for long-term outcomes 1