The Effect of Whole-Body Vibration on Patellofemoral Joint Pain After Anterior Cruciate Ligament Reconstruction

Anterior cruciate ligament (ACL) injury is a prevailing problem among sports participants, especially in non-contact sports. Studies have reported that 70-84% of ACL injuries are non-contact in nature, and movements like changing in direction while running, cutting and pivoting on a planted foot have resulted in a majority of ACL injuries. The influence of such injury can often be lifelong as it increases the patient's risk of developing degenerative musculoskeletal problems like osteoarthritis, and eventually affects the patient's quality of life. Even after ACL reconstruction (ACLR) surgeries, study has reported a 30-50% prevalence of developing patellofemoral joint (PFJ) pain in 1-2 years post-operation. The prevalence of patients developing PFJ osteoarthritis ranges from 14.8-26.5% for post-operation 2-5 years, and the percentage is even higher after more than 6 years post-operation.

Symptoms of PFJ osteoarthritis include swelling, notably reduced quadriceps strength, and pain on PFJ compression. Though the mechanism of how ACLR contributes to the development of PFJ osteoarthritis is not completely clear yet, inflammation and damage to PFJ articular cartilage and alteration of vastus medialis function, which is common after ACLR, might play a role in the development of PFJ osteoarthritis. A more than 30% decrease in quadriceps muscles strength had been reported by previous study. Decrease in size of rectus femoris, vastus medialis and vastus lateralis had also been observed. Though there was no significant difference in the vastus medialis/vastus lateralis, the decrease in strength and size of the quadricep muscles could possibly affect the stability of PFJ, increasing the chance of developing PFJ osteoarthritis.

Whole body vibration (WBV) therapy has been gaining attention as an effective method of training in recent years. More and more rehabilitation facilities have equipped the WBV machine and make this therapy more easily accessible. It has been proved to have a positive effect on improving muscle strength, muscle activities, muscle power and loading during drop jump. It has also shown effectiveness in decreasing osteoarthritic knee pain and enhancing knee function in chronic elderly knee osteoarthritis. Though the exact mechanism of the therapy is not yet very clear, it is believed that it is related to the "tonic vibration reflex". Skeletal muscles undergo small changes in muscle length during the vibration generated by the machine. It elicits the "tonic vibration reflex", activates muscle spindles and mediated neural signals. Another possible mechanism is that the vibration provides cortical stimulation and increases the corticomotor excitability. Both of them train the neuromuscular system to response faster, achieving the above-mentioned benefits.

There are different types of WBV machines, providing vertical, horizontal or pivoting vibration, and the amplitude and frequency of the vibration can be adjusted on the machine. Many studies have been conducted using different duration, amplitude and frequency of vibration. A "standard protocol" for WBV therapy is yet to be developed. Though duration of WBV may differ according to the effect of interest, several studies have had positive results with a 8 week WBV therapy in increasing muscle strength, proprioception, and post-ACLR knee functions. Further investigation on the underlying mechanism and possible application are to be continued to explore more possibilities with the WBV therapy.

Conservative and effective approaches for relieving pain are needed for knee osteoarthritis patients and, among these, pulsed electromagnetic fields (PEMF) are emerging with promising results. In vitro studies have demonstrated that PEMF therapy is effective in reducing chondrocyte apoptosis and MMP-13 expression of knee cartilage in ovariectomized rats and in favourably affecting cartilage homeostasis. PEMF was evaluated for function, a significant improvement was observed 8 weeks after initiation of treatment, and no significant association was found between the use of PEMF and the occurrence of adverse events.