Tissue Shrinkage With the Holmium:Yttrium Aluminum Garnet Laser: A Postoperative Assessment of Tissue Length, Stiffness, and Structure

The effect of laser energy on the length, stiffness, and structure of connective tissue was examined in a rabbit patellar tendon model. A holmium:yttrium-aluminum- garnet laser was used to deliver a calculated dose of laser energy (300 J/cm2) to one randomly selected patellar tendon in each of 13 adult New Zealand White rabbits. The contralateral patellar tendon was used as a control. Radiopaque markers were placed in the patella and tibial tuberosity to allow for patellar tendon length measurements (via standard lateral radio graphs) before and after laser application and at 4 and 8 weeks. Limbs were not immobilized during the post operative period. The tendons were harvested at 0 weeks (N = 7) and 8 weeks (N = 6) and evaluated for tensile stiffness, cross-sectional area, histologic changes, and electron microscopic appearance. The results demonstrated significant tendon shrinkage (6.6% ± 1.4%) after application of the calculated laser energy dose. However, tendon length had increased significantly beyond the immediate postlaser length at 4 weeks and beyond its original length by 8 weeks. At 8 weeks, the lased tendons were significantly less stiff with significantly greater cross-sectional areas than contralateral controls. There was generalized fibro blastic response throughout the entire lased tendon characterized by a marked increase in cellularity. There was also a change from the normal bimodal pattern of large- and small-diameter collagen fibers to a unimodal pattern with predominantly small-diameter fibers in the lased tendons. The tissue alterations seen in this study suggest that the biologic response of connective tissue to laser energy causes a further compromise in tissue integrity, beyond that attributed to the initial physical effects of the laser. These alter ations must be taken into consideration when deter mining postoperative rehabilitation of laser-modified tissues.