IKDC score, a subjective method of evaluating knee function, not only focuses on the evaluation of the patient’s symptoms and the stability of the knee joint, but also attaches importance to the assessment of knee motor function and is widely used because it allows accurate and comprehensive evaluation of knee function. After analyzing the correlation of 14 independent factors with postoperative IKDC classification, we found that sex, BMI, age of onset, symptoms duration and cartilage injury and its degree affect postoperative 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 sex is a favourable factor in many orthopaedic diseases, but its correlation with postoperative efficacy in patients with DLM is controversial. Ahn [5] et al. demonstrated that male sex was conducive to good postoperative outcomes by evaluating 260 patients with DLM. Through investigating 502 patients with DLM, we also found that male sex was a protective factor for good knee function (P=0.023, OR=1.702, 95% CI: 1.076-2.697). Compared with males, on one hand, females 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 females [13,14,17]. Thus, females are more susceptible to cartilage lesions and osteoarthritis [20] and being female is associated with poor postoperative clinical outcomes. However, Chen et al [11] and Kose et al [28] found that sex has no significant effect on postoperative result of DLM; this finding may be related to the small sample size in their study (n=39 cases, n=48 cases, respectively).
For symptomatic DLM, numerous studies have found that the earlier the age of onset is, the shorter is the duration of symptoms (especially <12 months) and that the earlier the surgical intervention is performed, the better is 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) are advantageous for postoperative efficacy. It has been reported that the younger age of onset and a shorter course of the disease are correlated with a lower risk of articular chondromalacia and damage caused by DLM lesions [2,17,31]. Moreover, early normalization of DLM morphology by surgery 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 arising 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 is associated with a higher likelihood of obtaining better postoperative results (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 suffer from articular cartilage lesions. High BMI has been shown to be the main risk factor for knee osteoarthritis, as obesity can lead to excessive compression of the meniscus and to loss of and pathological changes in the articular cartilage [16,30,45]. Hence, lower BMI is related to lower occurrence of articular cartilage lesions and knee osteoarthritis and thus better postoperative efficacy. Nonetheless, we found no effect of work intensity on the postoperative outcomes of symptomatic DLM (P>0.05); this may be because that work intensity is more reflects the activity of entire body rather than the pressure on the meniscus and cartilage of the knee.
The results of this study indicate that the absence of articular cartilage lesions (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 factors for postoperative recovery of knee function. 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 reflects 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 that is used to assess the severity of degeneration of the knee, we observed no relationship between KL grade and postoperative outcome, probably because X-rays are less sensitive in visualizing cartilage lesions [6] and because early joint space reduction is secondary not to articular cartilage thinning but to meniscal compression [38]. Moreover, Kose et al. [28] have shown that the combined medial meniscus tears did not affect postoperative outcomes, which is similar to the result of our study.
Generally, surgical methods for the arthroscopic treatment of symptomatic DLM by arthroscope includes saucerization (partial meniscectomy), saucerization with repair and total meniscectomy [2,9,25,47,49]. At present, the effect of surgical mode on postoperative outcomes is disputable. Ahn et al. [4] considered that partial meniscectomy with repair has a good efficacy in children with symptomatic DLM compared with total meniscectomy; this may be because partial meniscectomy with repair can prevent early degenerative changes of the 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 its abnormally fibrous structure, which may lead to adverse clinical effects. Considering the high cost and uncertain effectiveness of repair, repair of the abnormal anatomy in a torn DLM is not recommended [44]. Some scholars have not found the differences in 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 et al [32] and Wong and Wang [49] concluded that there was no significant difference among these three surgical methods in postoperative outcomes. In addition, a systematic review did not find a difference in postoperative outcome between partial meniscectomy with repair and saucerization, but these two methods have significantly improved outcomes over total meniscectomy [44]. In the present study, we discovered that the type of surgery does not affect the postoperative result. The discrepancies in the results obtained in these studies may exist because the choice of surgical method was affected by factors such as age at surgery, location of DLM tear, severity of the DLM tear and other factors and because the number of patients who underwent saucerization with repair and total meniscectomy were small.
Similar to the results of other studies [11,28,31], we also found that DLM type has no significant effect on postoperative efficacy. This may be why no significant differences were found in the incidence of articular cartilage lesions and postoperative discoid meniscus morphology among different types of discoid meniscus [15,17] [7].
Regarding DLM tear, some studies reported that DLM tears could lead to degeneration of the articular cartilage and osteoarthritis in the long term [15,51], thus contributing to poor postoperative outcomes. However, Ding et al [15] and Kose et al [28] concluded that discoid meniscus injuries are not correlated with articular cartilage lesions. In our study, DLM tears did not affect the postoperative effect, and no difference in cartilage damage was observed during arthroscopy with respect to whether or not DLM tear exist; this may be because that the majority of our patients with DLM tears had obvious symptoms and received timely diagnosis and treatment.
Concerning the influence of O'Connor tear type on the postoperative result, Chen et al [11] and Badlani et al [8] considered that radial tears lead to poor postoperative outcomes. Ahn et al [2] found that compared with other tear types, the duration of symptoms of horizontal meniscus tear, as a degenerative tear, is longer and the postoperative residual meniscus tissue is reduced and fragile, which may accelerate the radiological progression of postoperative KL grade 3/4 osteoarthritis. However, other studies observed that the duration of symptoms in cases of horizontal tear may not be significantly different from that in cases with other tear types [35, 43]. Here, we did not find a correlation of O'Connor tear type with postoperative effect; this may be attributed to the fact that the severity of cartilage damage is not related to the type of DLM tear [17] as well as to the fact that the difference in 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.
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 correlated with postoperative efficacy of symptomatic DLM; this may be due to the small number of patients with follow-up time over 120 months.
We acknowledge that our study has some limitations. At the final follow-up, the assessment of postoperative efficacy didn’t analyse the imaging changes but only evaluated the subjective functional parameters; thus, 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 should be further confirmed by prospective studies.