| BACKGROUND:Bone-tendon junction injury is not uncommon in clinic of Orthopaedics and Sports medicine and many tendons and ligaments are involved, such as Anterior cruciate ligament, Patellar tendon, Rotator cuff insertion, Plantaris muscle tendon and Achilles tendon. It is very difficult to healing between two different tissues:bone and tendon and lack of effective therapy. The single surgical repair often fails to get satisfactory insertion healing.It is well known that Magnetic fields have a positive effect on musculoskeletal system injury, involve fracture and spinal fusion because of its good effect on bone formation and regeneration. Combined Magnetic Fields (CMF), a new biophysical stimulation have been studied in musculoskeletal repair for its regulations on bone balance and bone metabolism. It has proved that bone formation has a close relation with bone to tendon healing.OBJECTIVE:The current study employed new and advanced evaluation technologies to investigate methods potentially to be indicated for enhancing the bone-tendon junction healing. The effect of Combined Magnetic Fields therapy to bone-tendon healing was observed to approach the therapy mechanism and it is provided the basis for application of Combined Magnetic Fields in clinic.METHODS:Standard partial patellectomy was conducted in the mature rabbits that were divided into the CMF treatment and placebo treatment groups (control group). Daily CMF therapy was delivered 3 days after partial patellectomy to the patella-patellar tendon (PPT) healing junction, and continuously up to the week 4,8,12 and 16 postoperatively, then the PPT complexes were harvested for evaluation. The evaluations involved:1) Radiography to study new bone formation and size; 2) Peripheral quantitative computational tomography (pQCT) for evaluation of bone mineral density (BMD), bone mineral content (BMC) and bone volume (BV) of PPT junction; 3) Tensile strength of quadriceps-patella-patellar tendon-tibia (QPPTT) comple, include Failure load, Stress at failure and Total energy, were tested; 4) The three dimensional microstructure of new formed bone in PPT junction was detected by VivaCT; 5) Histology with decalcified section for general histomorphology and healing process.RESULTS:1) It presented gradual outgrowth of new bone in PPT junction. The area and length of new bone in treatment groups increased with healing time. There are not significant difference in the area and length of new bone between treatment group and control at 4 weeks (p>0.05), but which of treatment group in 8,12 and 16 weeks are higher than control group, respectively (p<0.05); 2) BMC, BV and BMD of new bone in treatment groups have a gradual increase with the healing time. Exceptionally, BMD of new bone in 16 weeks has no significant difference compared with which in 12 weeks (p>0.05). There are not significant difference in BMC, BV and BMD of new bone between treatment group and the control at 4 weeks (p>0.05), but which of treatment group in 8,12 and 16 weeks are higher than control group (p<0.05), respectively; 3) The mechanical property, Failure load, Stress at failure and Total energy increased with the healing time. But there are not significant differences in Failure load and Stress at failure in treatment group compared between which of 16 weeks and 12 weeks (p>0.05). Compared between 8 weeks and 12 weeks in treatment, There is not significant differences in total energy, but the energy increased rapidly after 12 weeks in treatment group, which of 16 weeks is higher than 12 weeks (p<0.01). There are not significant difference in Failure load, Stress at failure and Total energy between treatment group and the control at 4 weeks (p>0.05), and the mechanical property of treatment group mentioned above, are high than control in 8,12,16 weeks (p<0.05), respectively. But there is not significant difference in Stress at failure compared between treatment and control at 16 weeks (p>0.05); 4) The newly formed bone could get a gradual bone formation and bone remodeling with the healing time, especially in treatment of 12 weeks and 16 weeks, the morphous of new bone graduated to a smooth and glossy structure, which almost gained an flat patellar-femur joint surface in image of VivaCT; 5) Histologically, the CMF treatment group showed improved tissue integration, characterized by trabecular bone expansion from the remaining patella and more rapid healing and remodeling of PPT junction. There are regeneration of fibrocartilage in treatment group at 12 weeks and 16 weeks.CONCLUSIONS:This is the first study to show that 30 minutes daily CMF therapy could accelerate the PPT junction repair in a partial patellectomy model in rabbits. It could through the following mechanism to achieve it:1) Accelerates new bone formation and maturation of new bone in PPT junction;2) Accelerates regeneration of fibrocartilage;3) Improves integration of bone and tendon interface and promotes bone remodeling and reconstruction of PPT junction.
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