The Healing Effects on the Biomechanical Properties of Joint Capsular Tissue Treated With Ho:YAG Laser: An In Vivo Rabbit Study Michele M. Schulz, M.D., Thay Q. Lee, Ph.D., Matthew D. Sandusky, B.S., James E. Tibone, M.D., and Patrick J. McMahon, M.D. Purpose: The objective of this study was to evaluate the healing response, after thermal treatment with a Ho:YAG laser, on the biomechanical properties of capsular soft tissue. Type of Study: Before and after trial. Methods: Forty-five New Zealand white rabbits were used in this study. A medial peripatellar retinacular thermal capsuloplasty using a Ho:YAG laser and a lateral peripatellar retinacular release was performed on 1 knee of each rabbit. The contralateral knee served as a control and had a lateral release of the retinaculum only. The temperature of the medial retinaculum was maintained at 55°C  5°C during treatment. The medial peripatellar retinaculum was evaluated at 0, 6, and 12 weeks postoperatively. Tensile testing of the medial retinaculum and a biomechanical assessment evaluating the structural and material properties were performed. Results: The ultimate load (force) of the medial retinaculum was 70%, 56%, and 84% of control at 0, 6, and 12 weeks, respectively, after the procedure. The stiffness (force/deformation) of the medial retinaculum was 83% of control at 0 weeks, 54% at 6 weeks, and 85% at 12 weeks. The ultimate stress (force/area) of the medial retinaculum also showed a significant reduction at 0 and 6 weeks postoperatively, 63% and 62% of control, respectively. By 12 weeks, the ultimate stress was 83% of control. Conclusions: Thermal treatment of the medial retinaculum with a Ho:YAG laser results in soft tissue with significantly diminished biomechanical properties after treatment. The results of this study suggest that a 12-week period of minimal stress on the capsular tissues should follow a thermal capsuloplasty procedure. Key Words: Thermal capsuloplasty—Healing—Biomechanics—In vivo rabbit study. A nterior glenohumeral joint instability has tradi- tionally been treated by open Bankart repair and tightening of the capsuloligamentous structures. Re- cent advances have made arthroscopic Bankart repair possible. 1-5 However, with current arthroscopic tech- niques, capsuloligamentous tightening can be difficult. Leather tanners have long known that heat, appropri- ately applied, decreases the size of a hide, and similar treatment may tighten capsuloligamentous structures. Accordingly, the length of human soft tissue decreases as its temperature is increased up to 70°C. 6-8 Thermal effects on soft tissue result from heat transfer that cause an ultrastructural alteration in its collagen fibril architecture. 9 Regardless of the method, raising soft tissue temperature to 50°C to 60°C results in denatur- ation of collagen elements. 6 Hyalinization and rean- nealing of the collagen can also occur. 7 This may be the reason for successful treatment of anterior gleno- humeral instability with a hot iron as described by Hippocrates in 460 BC. Today, heat can be applied to selective regions of intracapsular soft tissue using an arthroscopic ap- proach. The laser has been being used to assist in a variety of arthroscopic shoulder surgeries. 10,11 One From the Orthopaedic Biomechanics Laboratory, Veterans Af- fairs Medical Center, Long Beach; University of California, Irvine; and Kerlan Jobe Orthopaedic Clinic, University of Southern Cal- ifornia, Los Angeles, California (J.E.T.), U.S.A. Address correspondence and reprint requests to Thay Q. Lee, Ph.D., Orthopaedic Biomechanics Laboratory, Veterans Affairs Medical Center (151), 5901 E. 7th St, Long Beach, CA 90822, U.S.A. E-mail: tqlee@med.va.gov This is a US government work. There are no restrictions on its use. 0749-8063/01/1704-2518$0.00/0 doi:10.1053/jars.2001.19677 342 Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 17, No 4 (April), 2001: pp 342–347