IKDC score, the subjective knee evaluation form for evaluating knee function, not only focuses on the evaluation of the symptoms and stability of the knee joint, but also attaches importance to the assessment of knee motor function, which is widely used for its accurate and comprehensive knee function evaluation. After analyzing the correlation of 14 independent factors with the IKDC classification, we found that sex, BMI, age of onset, symptoms duration and cartilage injury and its degree have effects on IKDC classification, while preoperative work intensity, DLM type, DLM tear and its O'Connor type, combined medial meniscus injury, K-L grade, postoperative follow-up time and type of surgery did not significantly affect the postoperative result.
Male is a favourable factor for many orthopaedic diseases, but its correlation with postoperative efficacy of DLM is controversial. Ahn [5] et al. demonstrated that male was conducive to good postoperative outcomes by evaluating 260 patients with DLM. Through investigating 502 patients with DLM, we also found that male was a protective factor for good knee function (P=0.023, OR=1.702, 95% CI: 1.076-2.697). Compared with male, on one hand, female have increased rates of cartilage loss and progression of cartilage defects at the knee [20]. On the other hand, the knee articular cartilage volume is smaller and the Q angle is greater in female [13,14,17]. Thus, female is more susceptible to cartilage lesions and osteoarthritis [20] and associated with poor postoperative clinical outcomes. However, Chen [11] and Kose [28] found that sex have no significant effect on postoperative result of DLM, which may be related to the small sample size (n=39, n=48, respectively).
For symptomatic DLM, numerous studies have found that the younger the age of onset, the shorter the symptoms duration (especially <12 months), the earlier the surgical intervention, the better the prognosis [5,11,31,38,49]. We found that the age of onset <25 years, especially <14 years (P < 0.001, OR = 37.069; 95% CI: 7.822–55.147), and the symptoms duration <24 months (P < 0.001, OR = 3.254; 95% CI: 1.855–5.703) is advantageous for postoperative efficacy. It has been reported that the younger age of onset and the shorter course of disease is correlated with the lower risk of articular chondromalacia and damage caused by DLM lesion [2,17,31]. Moreover, the early normalizing DLM morphology by operation would not only increases the mobility of the meniscus, but also enhances the adaptability of the meniscus to the tibiofemoral surface, thereby reducing damage and degeneration of the meniscus and articular cartilage springing from excessive stress concentration [15,31,38]. In addition, shaping DLM in childhood may improve the dysplasia of the femoral condyle and the abnormality of the lower limb alignment, thus abating the risk of cartilage degeneration and delaying the occurrence and development of osteoarthritis [19, 24, 50].
This study found that BMI<18.5 kg/m2 have a higher possibility to obtaining better postoperative result (P = 0.026, OR = 3.016; 95% CI: 1.138–7.996). Fu [17] et al. found that patients with BMI >23.0 kg/m2 were more likely to articular cartilage lesions. High BMI is proved to be the main risk factor of knee osteoarthritis as obesity can lead to excessive compression of meniscus, and to loss and pathological changes of articular cartilage [16,30,45]. Hence, lower BMI is related to lower responsibility of articular cartilage lesions and knee osteoarthritis and thus better postoperative efficacy. Nonetheless, we didn’t find the effect of work intensity on the postoperative outcomes of symptomatic DLM (P>0.05), which may be because that work intensity is more reflect the activity amount of body, rather than the pressure on the meniscus and cartilage of the knee.
The results of this study indicate that no articular cartilage lesion (P<0.001, OR=6.379; 95% CI:2.545–15.975) and Outerbridge grade I (P=0.001, OR = 4.322; 95% CI: 1.412–13.277) are beneficial factor for postoperative knee function recovery. Outerbridge grade ≥II is an unfavourable factor for postoperative outcome (P=0.12, OR = 2.134). The clinical manifestations of an articular cartilage lesion may not be obvious in the short term, but most patients will eventually have knee degeneration associated with cartilage damage which leads to irreversible severe osteoarthritis drastically affecting knee function [46]. Hiroshi et al. [22] consider that the Outerbridge grade can directly reflect the severity of the cartilage lesions and is the decisive factor affecting the long-term outcomes after meniscal surgery. Although K-L grade is an imaging index to assess the severity of degeneration of the knee, we didn’t observe the relationship between KL grade and postoperative outcome, which probably because that X-rays are less sensitive in visualizing cartilage lesions [6] and that early joint space reduction is not secondary to articular cartilage thinning but to meniscal compression [38]. Moreover, Kose et al. [28] have shown that the combined medial meniscus tears didn’t exert impact on postoperative outcomes, which is similar to the result of our study.
Generally, surgical method treating the symptomatic DLM by arthroscope includes saucerization (partial meniscectomy), saucerization with repair and total meniscectomy [2,9,25,47,49]. So far, the effect of surgery mode on postoperative outcomes is disputable. Ahn et al. [4] considered that partial meniscectomy with repair has a good efficacy on children with symptomatic DLM compared with total meniscectomy, which may be because partial meniscectomy with repair can prevent early degenerative changes of joint [36]. However, Lee et al. [31] harbour the opposite opinion that residual discoid meniscus tissue is prone to degeneration and re-injury due to abnormally fibrous structure, which may lead to adverse clinical effects. Considering the high cost and uncertain effective of repair, repair of the abnormal anatomy in a torn DLM is not recommended [44]. Some scholars haven’t found the difference of clinical outcomes between partial meniscectomy and total meniscectomy in the short- or medium-term, but the clinical efficacy of partial meniscectomy is better than that of total meniscectomy in the long-term follow-up [2,3,9,25,34,44,47,50]. Conversely, Ikeuchi et al. [23] reported that the results of partial meniscectomy were significantly worse than those of total meniscectomy. However, Lee [32] and Wong et al. [49] concluded that there was no significant difference in the postoperative outcomes among these three surgical methods. Besides, a systematic review didn’t conclude the difference in postoperative outcomes between partial meniscectomy with repair and saucerization, but these two methods have a significantly improved outcomes over total meniscectomy [44]. In the present study, we discovered that the type of surgery isn’t affect postoperative result. The discrepancy of these studies’ results may be because that the choice of surgical method was affected by factors such as age at surgery, location of DLM tear, the severity of the DLM tear, etc. and that the sample size of saucerization with repair and total meniscectomy were small.
Similar to the result of other studies [11,28,31], we also found that DLM type have no significant effect on postoperative efficacy, which may be the reason that no significant difference existed in the incidence of articular cartilage lesions and postoperative discoid meniscus morphology among different types of discoid meniscus [15,17] [7].
As to DLM tear, some studies believed that DLM tears could lead to degeneration of articular cartilage and osteoarthritis in long term [15,51], thus relating to poor postoperative outcomes. However, Ding [15] and Kose [28] believed that discoid meniscus injuries are not correlated with articular cartilage lesions. In our study, DLM tear didn’t affect the postoperative effect and no difference of cartilage damage was observed under arthroscope in terms of whether DLM tear or not, which may be because that the majority of our patients with DLM tear have obvious symptoms and accept the timely diagnosis and treatment.
About the influence of O'Connor tear type on the postoperative result, Chen [11] and Badlani [8] considered that radial tears lead to poor postoperative outcomes. Ahn [2] found that compared with other tear types, duration of symptoms of horizontal meniscus tear, as a degenerative tear, is longer and the postoperative residual meniscus tissue of it is less and fragile, which may accelerate the radiological progression of postoperative KL grade 3/4 osteoarthritis. However, other studies observed that the symptoms duration of horizontal tear may not be significantly different from other tear types [35, 43]. Here, we did not find the correlation of O'Connor tear type with postoperative effect, which may be attributed to that the severity of cartilage damage isn’t related to the type of DLM tear [17] and that the difference of thickness between DLM and normal meniscus is not obvious even though removing a layer of horizontal meniscus tear as the discoid meniscus is thicker than the normal meniscus.
A longer follow-up is believed to be associated with more severe articular cartilage degeneration and clinical symptoms and worse knee joint function [31, 34]. In our study, the median follow-up time was 75.4 (range, 41~123; IQR, 33.7) months, and the final follow-up time was not an influencing factor for the postoperative efficacy of symptomatic DLM, which may be due to the small number of patients with follow-up time over 120 months.
We acknowledge that there are some limitations to our study. At the final follow-up, the assessment of postoperative efficacy didn’t analyse the imaging changes but only evaluated the subjective functional parameters, so there was no objective evaluation index that corresponded to the postoperative outcome. Moreover, this study is only a retrospective multivariate analysis, and the conclusions obtained need to be further confirmed by prospective studies.