Based on eight RCTs conducted in America and China, the application of PRP in meniscus repair might have a positive effect on patients’ VAS, Lysholm score, and the healing rate. However, these results should be interpreted carefully. We only included randomized, placebo-controlled trials. Most studies had explained their randomization methods, and only a few studies had explained allocation concealment methods. The experiments reported varying results. Most studies reported were adequate. There were a small number of cases where there was no standard deviation or graphic representation. There was significant heterogeneity in a part of our analysis, and we therefore used sensitivity analysis or subgroup analysis to address this concern.
Some studies[18, 21, 23] have shown that PRP cannot reduce the VAS score after meniscus repair, although our meta-analysis demonstrates that this was effective. This might be linked to the long follow-up events of these studies (12–42 months), while the follow-up time of the other two studies[20, 29] was 6 months, indicating that PRP may have a limited effect on alleviating long-term pain after meniscus repair. There was significant heterogeneity in the Lysholm score after the sensitivity analysis; the pooled analysis, after excluding a study[19], still suggested that PRP could improve the Lysholm score after meniscus repair. Analysis of the full text of Liu et al’s study[19] showed that the sex ratio, mean age, and random sequence generation between the two groups were not reported, which might be the reason for the high heterogeneity between this and other studies. In general, PRP could not further improve the healing rate of patients after meniscus repair, but due to the high heterogeneity of the included studies, we conducted a subgroup analysis and compared studies in America and China. We found that PRP could increase the healing rate in the subgroups, and there was also significant clinical heterogeneity between the subgroups. The reason for the above difference in healing rate in the subgroup analysis is likely related to the different follow-up time and types of PRP used in different countries. A few trials have adverse events and found that there may be no difference in the incidence of adverse events between participants receiving and not receiving PRP treatment.
All participants in the included studies were either American or Chinese; thus, this review is limited to representing the Chinese or American population. Many potentially confounding variables such as the age, sex, cause and of meniscus injury, categories of meniscus tear, surgical approach, and platelet content in PRP had not been reported in detail. Routine postoperative treatment varies by research included active/passive activities and weight training, among others.
The cause of meniscus tear is often related to the patient’s age. The most common cause of meniscus tear and/or deterioration in young and elderly patients is typically related to acute trauma to the joints and degenerative changes, respectively[31]. Due to the uniqueness of the meniscus structure, there are two different mechanisms for the healing of injuries. In the red area of the meniscus (vascular area), the abundant blood supply provides nutrients for mesenchymal cells to induce healing[32]. In the white area (avascular area), the healing of the meniscus depends on its own tissue repairability, which leads to difficult healing or even non-healing[33]. Meniscus repair is effective in treating meniscus injuries in the red area, with a healing rate as high as 90%, but it has a poor effect on injuries in the white area[34]. In young men with meniscus injuries, there are often simultaneous tears in two areas. Different types of meniscus tears and age-related causes of meniscus tears can lead to different healing abilities. At the same time, different meniscus repair methods have different repair capabilities. The all-inside meniscus repair systems are safer, faster, and more convenient and hence, more popular than other meniscus repair systems such as meniscus arrows, Fast-Fix, and RAPIDLOC meniscus repair[35]. In addition, different repair methods result in different movement of meniscus and sizes of popliteal hiatus, which further leads to different biomechanics and kinematics of the lateral knee joint compartment. However, the research included in this meta-analysis did not directly mention the age stratification of the participants, area of the meniscus where the tear was located, and the type of meniscus tear; furthermore, the meniscus repair methods used were also different. Therefore, more extensive subgroup analysis could not be performed to clarify the enhancement ability of PRP on different meniscus repair procedures; moreover, it was also not possible to judge whether PRP is age-related or tear type-related for enhanced meniscus repair procedures.
PRP is a platelet concentration obtained after centrifugation of peripheral blood, and its role in the repair of cartilage damage has gradually attracted attention in recent years. PRP mainly includes platelet-related leukocyte aggregates, high-density fibrous network structure, platelet-derived growth factor, transforming growth factor-β, insulin-like growth factor, epidermal growth factor, and vascular endothelial growth factor[36–37]. PRP can release a large number of anti-inflammatory factors to reduce local inflammation and can release a variety of growth factors to promote cell proliferation and regulate cell behavior[38]. In vitro studies have shown that chondrocytes and PRP exhibit a significant dose- and time-dependent increase in cell number and metabolic cell activity[39]. It has already been shown that even small variation in centrifugation settings can alter the content of the PRP product, which underlines the importance to describe the ingredients before applying PRP product[40]. However, only two studies[18, 23] verified the PRP contents by using enzyme-linked immunosorbent assays and blood analyzers. The PRP content used in various studies is inconsistent, which may be an important reason for the inconsistent clinical results. Thus, future studies should not only be carried out in a randomized placebo-controlled fashion but also characterize the applied PRP product to compare results revealed in different studies.
The recovery of knee joint function after meniscus repair requires long-term follow-up. Most studies generally chose a follow-up period of 6 months, and only some studies had a follow-up period of more than 12 months. This may also be the reason for the difference between the results of this review and previous studies[41–42]. According to the evidence in this review, PRP enhances the meniscus, and no side effects were reported, which indicates that PRP is generally well tolerated. However, we cannot draw conclusions about the safety of PRP based on limited data. Further multi-center, large-sample, and long-term follow-up clinical studies are needed to address these questions.
Future trials of PRP-enhanced meniscus repair need to establish standardized protocols and report in detail the application of randomization, allocation concealment procedures, and blinding. The basic characteristics of the participant, cause of the meniscus injury, types of meniscus tears, meniscus repair methods, and the PRP preparation method should also be listed in detail. Platelet content in PRP should also be tested. Conventional treatment regimens should be specified in each group. Outcome measurements should include not only VAS, Lysholm score, and healing rate for a longer follow-up time but also MRI data of the meniscus and serious adverse reactions. Upcoming trials of PRP-enhanced meniscus repair will also need to be conducted in people outside of China and the United States.