Abstract
The purpose of this study was to evaluate anterior cruciate ligament (ACL) function in knee osteoarthritis (OA) and clarify the relationship of MRI and arthroscopic findings with ACL function. Eighty patients with knee OA were evaluated for anterior knee joint laxity, Kellgren-Lawrence grade, MRI ACL grade, arthroscopic ACL grade, osteophytes, and meniscus tears. ACL function was significantly correlated with osteophyte scores and medial and lateral meniscus tears. The MRI ACL grade was strongly correlated with the arthroscopic ACL grade, and anterior knee joint laxity decreased as the grades increased, suggesting a decrease in the elasticity of degenerated ACLs.
Keywords: Anterior cruciate ligament, Total knee arthroplasty, Knee joint laxity, Osteoarthritis, MRI, Arthroscopy
1. Introduction
Despite significant improvements in knee function after total knee arthroplasty (TKA), patient satisfaction remains low.1 One of the causes of low patient satisfaction is anteroposterior knee instability associated with anterior cruciate ligament (ACL) deficiency. The ACL is essential in reproducing normal kinematics, such as femoral rollback2,3 and screw-home movement4 as well as the normal gait. In this context, preservation of the ACL in TKA appears to be necessary for the restoration of normal knee function. In fact, bicruciate ligaments retained in ACL-preserving TKA show high anteroposterior stability5; unfortunately, however, the native tibiofemoral kinematics are not fully restored6,7 One of the causes of failure to restore normal kinematics may be the degraded integrity of the ACL when exposed to osteoarthritic conditions. Therefore, it is important to determine preoperatively whether the ACL functions normally.
Previous studies have reported several methods for ACL evaluation in osteoarthritis (OA) of the knee: visual inspection,8 histological evaluation,9 magnetic resonance imaging (MRI),10 and plain radiographs.11 However, little is known about the function of the ACL in knee OA.12 A visually normal ACL does not necessarily imply histological integrity8; therefore, even if the ACL looks normal, whether it functions normally after TKA is another issue. A previous study showed that ACLs from osteoarthritic knees have abnormal tensile and viscoelastic properties compared with normal ones.13 Therefore, it is important to evaluate the function of an ACL exposed to osteoarthritic conditions. MRI is clinically advantageous because it allows noninvasive preoperative evaluation of the ACL and has been shown to be more sensitive than surgical inspection in detecting anterior cruciate ligament deterioration.10 However, the relationship between MRI findings and ACL function in OA is unclear. Elucidation of this relationship will be useful in determining the surgical indication for ACL-retaining TKA and might also lead to improvements in surgical outcomes.
The purpose of this study was to evaluate anterior knee joint laxity as an index of ACL function in OA and clarify the relationship of MRI and arthroscopic findings with anterior joint laxity. In addition, osteophyte formation, joint space narrowing, and meniscus tears were also evaluated, as these factors may affect anterior knee joint laxity. The hypothesis was that anterior knee joint laxity would decrease according to ACL degeneration.
2. Materials and methods
2.1. Patient selection
This study was approved by the institutional review boards of the authors’ institutions. The surgical database was searched to identify patients diagnosed with OA between October 2016 and December 2017; 148 consecutive relevant patients were listed. These patients had undergone arthroscopic surgery for degenerative meniscus tears with or without osteotomies around the knee. The exclusion criteria were rheumatoid arthritis (RA), a history of knee injuries (e.g., ACL injury and meniscus injury) and previous surgeries (e.g., osteotomy around the knee, meniscus surgery, and ACL reconstruction). One patient with RA, 59 with a history of previous surgery, and eight with a history of knee injury (seven ACL injuries and one tibial plateau fracture) were excluded according to the criteria. Ultimately, the study included 80 patients (32 males and 48 females) with a mean age of 62.3 ± 8.2 years (range: 40 to 77) (Table 1).
Table 1.
Patient characteristics (n = 80).
| Value | ||
|---|---|---|
| Age, mean ± SD (range) | 62.3 ± 8.2 (40 − 77) | |
| Sex (male/female) | 32/48 | |
| Anterior knee joint laxity, mean ± SD (range) | 6.8 ± 2.3 (3 − 16) | |
| Osteophyte score, mean ± SD (range) | 10.8 ± 8.7 (0 − 32) | |
| Medial meniscus tear (Yes/No) | 68/12 | |
| Lateral meniscus tear (Yes/No) | 17/63 | |
| Grade | N (%) | |
| Kellgren-Laurence grade | 0 | 0 (0.0) |
| 1 | 13 (16.2) | |
| 2 | 21 (26.2) | |
| 3 | 30 (37.5) | |
| 4 | 16 (20.0) | |
| MRI ACL grade | 0 | 43 (53.7) |
| 1 | 17 (21.2) | |
| 2 | 20 (25.0) | |
| 3 | 0 (0.0) | |
| 4 | 0 (0.0) | |
| Arthroscopic ACL grade | 0 | 38 (47.5) |
| 1 | 21 (26.2) | |
| 2 | 16 (20.0) | |
| 3 | 3 (3.7) | |
| 4 | 0 (0.0) |
2.2. Radiographs
Knee radiographs with the Rosenberg view were assessed for Kellgren-Lawrence (K-L) grading.14
2.3. Evaluation of anterior knee joint laxity
Anterior knee joint laxity was evaluated preoperatively by the anterior tibial translation (ATT) test with a KS Measure Arthrometer (Sigmax Medical, Tokyo, Japan) after the induction of anesthesia.15, 16, 17 The relative movement between the patella and tibial tubercle sensor pads was recorded with 30 pounds of force at 30° of flexion. The same observer performed all of the tests to eliminate interobserver variation. The measurements were performed three times, and the mean value of the latter two measurements was used.
2.4. Arthroscopic ACL grading
The ACL was arthroscopically graded according to a previously described system.16 In this system, ACL status was graded from 0 to 4 as follows: grade 0 (an intact ACL), grade 1 (partial loss of synovial coverage), grade 2 (complete loss of synovial coverage), grade 3 (partial tear of the substance), and grade 4 (complete tear of the substance).
2.5. MRI ACL grading
The ACL was graded from 0 to 4 as follows: grade 0 (an intact ACL with low intensity throughout), grade 1 (partial intermediate intensity), grade 2 (intermediate intensity throughout, with or without an ill-defined and/or thickened ligament), grade 3 (a partial tear with a thinned and/or ill-defined ligament), and grade 4 (an ACL tear). An ill-defined and thickened ligament with increased signal intensity on proton-density-weighted fat-suppressed sequences indicates mucoid degeneration.18
2.6. Osteophyte score
Osteophytes were scored according to the scoring standard for osteophytes in the MRI Osteoarthritis Knee Score (MOAKS) system.19 The locations where osteophytes were evaluated were along the trochlea; the central and posterior margins of the femoral condyles and tibial plateaus; and along the medial, lateral, superior and inferior margins of the patella. Grades were assigned according to size, as follows: grade 0, none; grade 1, small; grade 2, medium; and grade 3, large (total score range: 0 to 36 points).
2.7. Reproducibility of measurements
The K-L grade, arthroscopic ACL grade, and MRI ACL grade were assessed by two independent orthopedic surgeons, and the mean of each value was used as the final value. For the assessment of arthroscopic ACL grades, surgical videos were used. There was an interval of 4 weeks or more between the two reproducibility readings. The intraobserver and interobserver reliabilities for the grades were expressed as intraclass correlation coefficients (ICCs; two-way mixed effects model) that varied from 0 (no agreement) to 1 (total agreement).
2.8. Statistics
Statistical comparisons were performed with SPSS (version 25.0; SPSS, Chicago, Illinois) and GraphPad Prism (ver. 8.4.3, GraphPad Software, San Diego, CA). Paired t-tests were used to compare two dependent parameters. A one-way factorial analysis of variance (ANOVA) with a post hoc Dunn's multiple comparison test was performed to evaluate the differences in anterior knee joint laxity according to each of the following: K-L grade, MRI ACL grade, and arthroscopic ACL grade. Spearman's correlation test was used to assess the correlation between anterior knee joint laxity and other parameters. Multiple regression analysis was conducted to identify factors associated with anterior knee joint laxity.
3. Results
3.1. Anterior knee joint laxity according to sex, K-L grade, arthroscopic ACL grade, MRI ACL grade, and meniscus tears
The baseline characteristics of the patients are shown in detail in Table 1. There was no significant difference in anterior knee joint laxity between sexes. Anterior knee joint laxity was significantly lower in K-L grade 4 than grade 1 or 3 knees (P < 0.05 for each comparison, Fig. 1). Although there were no significant differences in anterior knee joint laxity according to arthroscopic or MRI ACL grades, there was a trend toward reduced anterior laxity as grades increased (Fig. 1). The medial meniscus tear group showed significantly higher anterior knee joint laxity than the non-tear group, whereas the lateral meniscus tear group had significantly lower laxity than the non-tear group (p < 0.05 and < 0.001, respectively, Fig. 2).
Fig. 1.
Anterior knee joint laxity according to the K-L grade, arthroscopic grade, and MRI ACL grade. *; p < 0.05.
Fig. 2.
Anterior knee joint laxity according to meniscus tears. MM; medial meniscus, LM; lateral meniscus. *; p < 0.05, **; p < 0.001.
3.2. Correlation analysis
The results of the correlation analysis related to anterior knee joint laxity are summarized in Table 2. Anterior knee joint laxity was shown to be significantly correlated with the osteophyte score (r = −0.2634, p = 0.019) and the presence of medial meniscus tears (r = 0.232, p = 0.039) and lateral meniscus tears (r = −0.3876, p < 0.001) (Table 2). The arthroscopic ACL grade was moderately correlated with the MRI ACL grade (r = 0.632, p < 0.0001, Fig. 3).
Table 2.
Spearman correlation analysis with anterior knee joint laxity (n = 80).
| Parameter | Correlation coefficient | P value |
|---|---|---|
| Age | 0.0401 | 0.725 |
| Sex | 0.0246 | 0.829 |
| Kellgren-Laurence grade | −0.204 | 0.071 |
| MRI ACL grade | −0.148 | 0.192 |
| Arthroscopic ACL grade | −0.156 | 0.167 |
| Osteophyte score | −0.2634 | 0.019 |
| Medial meniscus tear | 0.232 | 0.039 |
| Lateral meniscus tear | −0.3876 | <0.001 |
Fig. 3.
Correlation between the arthroscopic ACL grade and MRI ACL grade. Spearman's correlation analysis showed a moderate correlation between the arthroscopic ACL grade and MRI ACL grade (r = 0.632, p < 0.0001).
3.3. Multiple regression analysis
With a dependent variable of anterior knee joint laxity and independent variables of age, sex, BMI, K-L grade, MRI ACL grade, arthroscopic ACL grade, osteophyte score, and lateral and medial meniscus tears, only lateral meniscus tears were recognized as an independent explanatory factor for a decrease in anterior knee joint laxity (β = −0.347, p = 0.002).
3.4. Reproducibility of measurements
The intraobserver and interobserver reliability values for K-L grades were found to be 0.98 and 0.95, respectively, and those for arthroscopic ACL grades were 0.97 and 0.96, respectively; the corresponding values for MRI ACL grades were 0.93 and 0.88, respectively.
4. Discussion
The most important finding of this study was that ACL function in OA was significantly associated with K-L grades, osteophyte scores, and medial and lateral meniscus tears but not arthroscopic ACL grades or MRI ACL grades. Lateral meniscus tears were an independent explanatory factor for a decrease in anterior knee joint laxity.
Many of the previous studies that evaluated ACL degeneration in OA were performed in end-stage OA.20 However, OA knees suitable for ACL-retaining TKA may be in a relatively early stage of OA without severe deformities, large osteophytes, or restricted ranges of knee motion.21 In this context, the cohort of this study was appropriate for a study regarding the ACL in ACL-retaining TKA. In the current study, 80% of the patients had knees of K-L grade 3 or lower without a limited range of motion; thus, they were candidates for the procedure. Even when ACLs appear normal macroscopically, histological evaluation may show degenerative changes, including collagen fiber degeneration, mucoid degeneration, and cystic changes.22 Histological examination might be the most sensitive method for evaluating ACL integrity,8 but it is not useful for decision making regarding the indications for ACL-retaining TKA in terms of sacrificing the ACL. On the other hand, MRI, which can depict and characterize subtle soft tissue and ligament degeneration, has high potential as a method for determining surgical indication because it can be performed noninvasively before surgery.18 In the current study, the MRI ACL grade showed a high correlation with arthroscopic ACL grade, suggesting that the loss of synovial coverage of the ACL is one of the findings of ACL degeneration. Synovial coverage of the ACL can also be evaluated with dynamic contrast-enhanced MRI.23 The addition of contrast allows assessment of the vascular supply in the synovial membrane by measuring the enhancement of the ACL. Thus, dynamic contrast-enhanced MRI may be a more powerful option to simultaneously evaluate the degeneration of the ACL substance and the superficial synovial membrane to inform decisions on ACL-retaining TKA.
Although the trend was not statistically significant, anterior knee joint laxity decreased as the MRI and arthroscopic ACL grades increased (Fig. 1). Because higher MRI and arthroscopic grades indicate ACL degeneration, it is suggested that anterior knee joint laxity decreases as ACL degeneration progresses. In close agreement with our results, several studies have also suggested that the ACL shows a decrease in laxity as OA progresses.13, 24 In the current study, anterior knee joint laxity was positively correlated with medial meniscus tears. Because the medial meniscus plays a crucial role in the anteroposterior stability of the knee joint,25, 26, 27 medial meniscus tears may have a greater effect on anterior knee joint instability than degraded ACLs. On the other hand, anterior knee joint laxity was negatively correlated with osteophyte scores and lateral meniscus tears. Osteophytes cause intercondylar notch stenosis and produce a shear force on the ACL during knee motion, which eventually induces ACL degeneration.28, 29 In knee OA, a stretched capsule and limited range of knee motion due to osteophytes may affect anterior joint stability. Therefore, it remains necessary to investigate whether the cause of decreases in anterior knee joint laxity is due to degenerated ACLs due to osteophytes, the effect of capsular contracture, or the limited range of knee motion secondary to osteophytes. Dynamic MRI is another option to evaluate ACL function. Guenoun D et al.30 found that dynamic MRI using an open MRI scanner showed significant ACL elongation in vivo during bending. They showed that the normal ACL elongates by an average of 3.85 mm between hyperextension and 90° flexion. Thus, a dynamic MRI may be useful in assessing degeneration and inelasticity of the ACL. Lateral meniscus tears showed the strongest correlation with anterior knee joint laxity and were the only predictor of laxity, as shown in the regression analysis. As with osteophytes, further research should investigate whether lateral meniscus tears induce ACL degeneration and eventually decrease anterior knee joint laxity or whether lateral meniscus tears directly induce joint capsular contracture. In summary, this study showed that a higher K-L grade, the presence of lateral meniscus tear, and abundant osteophyte formation are associated with decreased anterior knee joint laxity, indicating decreased function of the ACL. Surgeons need to consider these factors when determining whether the ACL functions normally for ACL-retaining TKA.
There were several limitations to this study. Although we measured anterior knee joint laxity as ACL function, some osteoarthritic changes, such as osteophyte formation, joint capsular contracture, and degenerated articular cartilage, may affect anterior knee joint laxity. These confounding factors remain to be eliminated. In addition, biomechanical study is necessary to evaluate the biomechanical properties of the ACL in addition to anterior knee joint laxity. Next, we did not assess whether the MRI ACL grade reflected the histological degeneration of the ACL. Future studies should assess the validity of the MRI ACL grade by comparing it with the histological evaluation.
5. Conclusions
ACL function was correlated with osteophyte scores and medial and lateral meniscus tears but not the MRI ACL grade or arthroscopic ACL grade. The MRI ACL grade was strongly correlated with the arthroscopic ACL grade, and both grades showed a similar trend: anterior knee joint laxity decreased as the grades increased, suggesting a decrease in the elasticity of degenerated ACLs.
Declaration of competing interest
None of the authors has any conflict of interest to declare.
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