Recent scientific studies evaluating laser energy for tissue welding and thermokeratoplasty have demonstrated that the application of laser energy at non-ablative levels can alter collagen's structural and biochemical properties. The application of non-ablative laser to the human shoulder joint capsule in patients with glenohumeral instability has been found to enhance stability of the joint. Based on the collective findings of these studies, we hypothesized that thermal modification of dense collagenous tissues such as joint capsule, ligament, and tendon can be achieved by applying non-ablative laser energy. The purpose of this study was to evaluate the effect of laser energy at non-ablative levels on joint capsular mechanical properties in an in vitro rabbit model. Twelve mature New Zealand white rabbits, ranging from 3.73 to 5.33 kg (4.49 +/- 0.44; mean +/- SD), were used for this experiment. Animals were euthanized and two 5 mm X 20 mm specimens were collected from the medial and lateral portion of the femoropatellar joint of each rabbit under a dissecting microscope; therefore four specimens were collected from each rabbit (right medial, right lateral, left medial, left lateral). Specimens were divided into four groups using a randomized block design; a control group and 3 laser power settings (5 watts (5 W), 10 watts (10 W), 15 watts (15 W)). Laser energy was applied using the Ho:YAG laser in four transverse passes across the tissue at a velocity of 2 mm/sec and distance from the tip of the handpiece to the synovial surface of the specimen set at 1.5 mm in a 37 degree(s)C tissue bath of lactated Ringer's solution. Forty-eight specimens (n equals 12) were mechanically tested to determine single cycle structural properties (stiffness) and viscoelastic properties (% relaxation) before and after laser treatment. Shrinkage of the tissue and the loads required to return specimens to their original length were recorded after laser treatment. The application of laser energy resulted in 9%, 26%, and 38% reduction in capsular tissue length for the 5 W, 10 W, and 15 W group, respectively. Tissue shrinkage was significantly and strongly correlated with energy density (p < 0.0001; R2 equals 0.80). Laser energy caused a significant decrease in tensile stiffness only in the 10 W and 15 W groups (p < 0.05). There was a significant but weak correlation between energy density and post-laser stiffness values (p < 0.0001; R2 equals 0.34). Laser energy did not change the relaxation properties of capsular tissue at any energy density when compared to pre-laser values (p > 0.05; (Delta) equals 29% at power equals 0.8). There was no significant correlation between energy density and relaxation properties of the tissue (p > 0.05). The loads required to return specimens to their original length were significantly lower for the 5 W group (3.6 +/- 5.1 N) than for the 10 W (15.0 +/- 6.2 N) and 15 W (14.0 +/- 5.2 N) groups. There was a significant correlation between energy density and the load required to return the specimen to its original length (p < 0.0001; R2 equals 0.58). This study demonstrated that significant capsular shrinkage can be achieved with the application of non-ablative Ho:YAG laser energy without detrimental effects on the viscoelastic properties of the tissue, although at higher energy densities, laser energy did lessen capsular stiffness properties.