The purpose of this study was to focus on the possible biomechanical testing of patellar instable patient before and after thermal shrinkage of medial parapatellar capsule. Our test set-up allowed us to record force applied and lateral displacement of patellar during a simulated physical exam on both healthy subjects and cadaveric knees. The force applied, lateral displacement of the patella and the stiffness of the medial parapatellar capsule were compared among the healthy subjects, cadaveric knees before thermal shrinkage, cadaveric knees after thermal shrinkage and open surgery. The test set-up was capable to quantify the force applied, lateral displacement of the patella during physical exam of healthy subject and can be used in future studies of evaluating the effectiveness of thermal shrinkage of medial parapatellar capsule in patients with recurrent dislocation of patella. The study did not find significant changes of the medial parapatellar capsule in resisting lateral force.
Patellar kinematics of cadaveric knees has been studied extensively. Three-dimensional patellar movement during knee flexion and extension has been studied in vitro using cadaveric knees . The medial and lateral translation of the patella was about 4 mm medial from full extension to 20° flexion, about 7 mm lateral from 20° to 90° flexion. The initial 4 mm medial translation is very important to prevent patella from dislocation laterally. A tight medial parapatellar capsule may contribute the medial translation at initial knee flexion. Thermal shrinkage of medial parapatellar capsule may improve its stiffness and capacity in resisting lateral dislocation of the patella. Although the basic science of laser- and radiofrequency-induced capsular shrinkage has been studied extensively [15–22], in recent prospective studies of shoulder with a wide spectrum of diagnoses, the effectiveness of thermal capsulorrhaphy has been mixed [7–9]. It is important to quantify the effectiveness of thermal capsulorrhaphy in clinic. This study investigated the feasibility of biomechanical testing of the medial parapatellar capsule in living subjects and cadaveric knees. The test set-up could be used on patients with recurrent dislocation of patellar before and after thermal capsulorrhaphy in future.
The medial patellofemoral ligament (MPFL) plays a major role in patellar stability [24, 25]. The MPFL consists of a thickened band of tissue originating from the medial epicondyle and inserting on the superior half of the patella. Nomura et al measured the increased laxity resulting from cutting the MPFL . They applied a 10 N tension on the quadriceps and a 10 N lateral displacing force. The lateral displacement of the patella increased from 6 mm for the intact knee to 13 mm after cutting the MPFL. Hautamaa et al applied 9 N to the quadriceps and a lateral displacing force of 22 N to the patella . They found a mean patellar displacement of 9 mm, which is similar to our results. We applied a tensile force of 18 N to the quadriceps to simulate the tension at rest. Our applied force to cadaveric knees averaged about 23 N which is similar to their lateral displacing force of 22 N. The thermal effect on the MPFL is unknown. In this study, we did not monitor the temperature change along the depth of the tissue.
Six degrees of freedom patellar tracking during first 15° voluntary knee flexion has been studied in vivo using optoelectronic motion capture system with a small patellar clamp . In a pilot study we followed their procedure on 3 healthy subjects and found the lateral translation measure in six degrees of freedom was almost identical from LVDT sensor, the other two translations were less than 2 mm, and three rotations was less than 3 degrees. But the procedure was very complicated and the patellar clamp was difficult to stay still relative to the patella. In a full extension position a subject lying on an exam table was much easier to be relaxed than a flexed knee position, which produces minimal influence to the lateral displacement by the quadriceps. Cadaveric knee data also demonstrated similar lateral displacement and stiffness between full extension and 20° knee flexion. Both male and female subjects demonstrated similar lateral displacements during physical exam, though higher forces were applied to the males. Our results also show that there were no significant mechanical differences between live subjects and fresh cadavers. This data may be useful in estimating the probable effects of thermal shrinkage on the knee capsule in patients.
Effectiveness of thermal shrinkage has been studied at length in animal models [15, 16, 22, 29]. Studies using animal specimens found ultrastructural alterations including a general increase in cross-sectional fibril diameter and loss of fibril size variation. Thermally induced ultrastructural collagen fibril alteration is likely the predominant mechanism of tissue shrinkage caused by application of radiofrequency energy. Over the last two to three years, arthroscopic thermal capsulorrhaphy for treatment of shoulder instability has undergone vigorous examination [7, 30–38]. Although the short-term outcomes of shoulder capsule shrinkage did not show significant difference than those without capsular shrinkage, long-term outcomes of thermal shrinkage for baseball pitchers are much better. Dugas and Andrews  reported an approximate 20% improvement in the rate of return to play with the addition of thermal capsular shrinkage to traditional treatments. Reinold et al.  studied the return-to-competition rate and functional outcome of overhead athletes following arthroscopic thermal-assisted capsular shrinkage. They followed 130 overhead athletes and found 87% successfully returned to competition with good-to-excellent long-term results. However, recently there are reports of glenohumeral chondrolysis after shoulder arthroscopy with thermal capsulorrhaphy [32, 41, 42]. Excessive heat from the procedure may have led to chondral damage and further research is needed to prevent this complication.
Coons and Barber treated 53 knees with a combination of capsule shrinkage and lateral release and followed them for an average of 53 months . Outcome was measured using the Lyscholm and Fulkerson knee scores, physical exam and the visual analog score. Subjectively, 90% of the patients reported excellent or good results. These results suggested that thermal capsule shrinkage may be valuable in treating the instable patella. However no detail of the thermal capsule shrinkage was reported regarding the temperature and power used. According to animal studies the amount of shrinkage potential was directly related to the temperature of the probe, the time of application, and the tissue quality [4, 20, 21, 43]. We used the intact cadaver knee joint and applied 65°C 40 watts as suggested by the manufacture. We did not find post-treatment changes of lateral displacement and stiffness. This could be due to the old age of the specimens. As a result of decreasing quantities of heat-sensitive bonds between type 1 collagen molecules, the potential for shrinkage decreases with increasing age. The decreased tissue stiffness of isolated tissue by thermal shrinkage may be accountable for unchanged displacement . The treated tissue began to show signs of healing by 6 weeks and the tissue stiffness returned to normal by 12 weeks .
Mini-open medial reefing and arthroscopic lateral release have been used to treat recurrent patellar dislocation [23, 44–46]. Good clinical outcomes have been reported with improved knee mobility and daily function. After mini-open medial reefing, the lateral displacement was reduced to 53% and the stiffness against the lateral force was over two times when compared with pre-reefing data. Our results confirmed the immediate effectiveness of medial reefing and matched these reported clinical data.
The limitations of this study are that the specimens were fresh-frozen and thawed over 24 hours, and they came from people over 60 years of age and the patella may not be instable. The influences of freezing on tissue response to thermal energy may be more significant than we expected. The temperature and power applied were set by the manufacture for clinical application. It was not the purpose of this study to investigate the influences of applied temperature and power, which have been done extensively in animal studies. Although we evaluated several knee flexion angles in our pilot study, for the full study we only tested at full extension of the knee to reduce the number of factors affecting our data collection in future clinical studies.
This study measured the immediate effect of applying thermal energy to the medial parapatellar capsule of human cadaver knees. We found that the fresh-frozen cadaver knees were similar in biomechanical properties of lateral displacement and stiffness to healthy young adults. The application of thermal energy to the medial capsular structures of human cadaver knees produced no statistically or clinically appreciable differences in medial structure stiffness compared to pre-treatment values. This study suggests that there is no need to test the patellar stability right after treatment for future clinical studies. The testing protocol worked fine with human subjects and cadaver knees. After proper post-operative immobilization and tissue healing, it is possible that this procedure may provide a reasonable alternative to open surgery for the treatment of patellar instability. Further clinical study is needed to investigate the long-term effect of thermal modification on the knee capsule.