What happens to the patellar tendon after removing a third during Bone-Patella Tendon-Bone (BPTB) Anterior Cruciate Ligament (ACL) reconstruction?

Patellar Tendon Healing After BPTB ACL Reconstruction

After removing a third of the patellar tendon during bone-patella tendon-bone (BPTB) ACL reconstruction, the tendon undergoes compensatory enlargement through scar tissue formation, which enables recovery of tendon stiffness despite reduced tissue quality. 1

Healing Process and Structural Changes

The patellar tendon undergoes several changes after harvest for BPTB grafts:

1. Compensatory Enlargement

   • The cross-sectional area (CSA) increases by approximately 21% compared to uninjured tendons 1
   • This enlargement is primarily due to scar tissue formation at the harvest site

2. Mechanical Properties

   • Tendon stiffness recovers to levels comparable to the uninjured side
   • Young's modulus (material quality) remains reduced by approximately 24% 1
   • The newly formed tissue has inferior mechanical properties but sufficient quantity to restore overall function

3. Timeline of Recovery

   • Early mobilization improves knee flexion and extension range of motion without compromising stability 2
   • Patellofemoral pain reduction is significant (from 35% to 8%) with early weight-bearing compared to non-weight-bearing protocols 2

Rehabilitation Considerations

The rehabilitation approach must balance strengthening with donor site protection:

1. Early Phase (0-2 weeks)

   • Isometric quadriceps exercises and straight leg raises can be safely prescribed during the first 2 postoperative weeks 2
   • These exercises promote faster recovery of knee range of motion without compromising stability

2. Intermediate Phase (3-6 weeks)

   • Leg press exercises can be started at 3 weeks to improve subjective knee function and functional outcomes 2
   • Both open and closed kinetic chain exercises are recommended for regaining quadriceps strength 2
   • Caution: Open kinetic chain exercises may induce more anterior knee pain compared to closed kinetic chain exercises 2

3. Advanced Phase (6+ weeks)

   • Eccentric training may improve functional outcomes and psychological readiness to return to sport 2
   • A combination of eccentric and plyometric exercises is more effective for improving balance and functional activities 2

Potential Complications

1. Patellar Fracture

   • Rare but significant complication during rehabilitation 3
   • Can occur during eccentric knee extension exercises or maximum voluntary isometric contractions 3
   • The American Academy of Orthopaedic Surgeons (AAOS) recommends balancing quadriceps strengthening with protection of the donor site 4

2. Patellar Tendon Rupture

   • Can occur both in early postoperative period and years later 5
   • Partial ruptures may be managed nonoperatively if extensor mechanism remains competent 6
   • Complete ruptures typically require surgical repair

Clinical Implications

1. Graft Selection Considerations

   • AAOS suggests that surgeons may favor hamstring grafts to reduce the risk of anterior or kneeling pain 2, 4
   • BPTB grafts may be preferred when prioritizing lower rates of graft failure or infection 2

2. Long-Term Outcomes

   • Despite donor site changes, long-term outcomes of BPTB ACL reconstruction are generally favorable 7
   • The majority of patients maintain improved function and stability at 5-year follow-up 7

Practical Recommendations

1. Balance rehabilitation goals between improving quadriceps strength and protecting the donor site
2. Monitor for anterior knee pain during rehabilitation, particularly with open kinetic chain exercises
3. Implement early mobilization to improve range of motion and reduce patellofemoral pain
4. Progress gradually from isometric to concentric to eccentric exercises
5. Combine eccentric and plyometric training in later rehabilitation phases for optimal functional outcomes

The patellar tendon's ability to heal and adapt through compensatory enlargement demonstrates the remarkable capacity for recovery, though clinicians should remain vigilant about potential complications during the rehabilitation process.