Sonographic Imaging of Meniscal Subluxation in Patients with Radiographic Knee Osteoarthritis

ORIGINAL ARTICLE Sonographic Imaging of Meniscal Subluxation in Patients with Radiographic Knee Osteoarthritis Chun-Hung Ko, 1 Kam-Kong Chan, 1 Hui-Ling Peng 2 Background/Purpose: This study was undertaken to describe the sonographic features of meniscal subluxation in the weight-bearing position and to determine any association between meniscal subluxation and radiographic osteoarthritis. Methods: In total, 238 knees with symptoms were examined successfully with weight-bearing anteroposterior and lateral radiographs and high resolution ultrasonography. The radiographs were examined to determine whether participants had radiographic osteoarthritis, graded using the Kellgren-Lawrence Scale. The degree of subluxation of the medial meniscus in each knee was measured using high resolution ultrasound with a 10-MHz linear transducer, at the level of the medial collateral ligament in weight-bearing condition. The degree of subluxation was compared in knees with the presence or absence of radiographic osteoarthritis using Student s t test. Additional analysis between knees with early and advanced radiographic osteoarthritis was also performed. Results: Meniscal subluxation for knees with (n = 141) and without (n = 97) radiographic signs of osteoarthritis were 4.3 ± 1.9 mm and 0.7 ± 0.6 mm, respectively. The difference was highly significant (p < 0.001). After age adjustment, the medial meniscal subluxation of age-matched subjects were 4.8 ± 1.7 mm for knees with radiographic osteoarthritis (n = 43) and 1.0 ± 0.8 mm for knees without such changes (n = 43). The difference between the two groups was still significant (p < 0.001). The greatest meniscal subluxation was seen in knees with advanced radiographic signs of osteoarthritis; no knee with osteoarthritic changes on radiographs had an undisplaced meniscus. Conclusion: Meniscal subluxation is a prominent feature on weight-bearing sonographic imaging in patients with radiographic osteoarthritis and could be considered as a risk factor for the development of knee osteoarthritis. By using musculoskeletal ultrasonography, one can detect this occult meniscal derangement early before the appearance of radiographic signs of osteoarthritis. [J Formos Med Assoc 2007;106(9):700 707] Key Words: knee, medial meniscus, meniscal subluxation, osteoarthritis, ultrasound The menisci of the knee are C-shaped wedges of fibrocartilage that lie between knee joint surfaces of the femur and the tibia. Previously, it was postulated that the meniscus functioned mainly as a space filler and a means whereby lubrication of the joint was enhanced. However, Fairbank 1 suggested that the meniscus probably has a weightbearing function as well. He based his opinion on a radiographic study of patients who had undergone meniscectomy. The three signs (so-called 2007 Elsevier & Formosan Medical Association....................................................... 1 Department of Orthopedics and Traumatology, Cardinal Tien Hospital, and 2 Department of Diagnostic Radiology, Shin Kong Wu Ho-Su Memorial Hospital, Taipei, Taiwan. Received: December 11, 2006 Revised: January 9, 2007 Accepted: June 5, 2007 *Correspondence to: Dr Hui-Ling Peng, Department of Diagnostic Radiology, Shin Kong Wu Ho-Su Memorial Hospital, 95 Wen Chang Road, Shihlin District, Taipei 111, Taiwan. E-mail: vivian12@ms2.hinet.net 700 J Formos Med Assoc 2007 Vol 106 No 9

Meniscal subluxation and osteoarthritis Fairbank s signs) he described were: an anteroposterior ridge projecting downward from the femoral condyle; generalized flattening of the marginal half of the femoral articular surface; and narrowing of the joint space. These signs are a subset of the radiographic signs of osteoarthritis (OA) and can be seen in knees that have not undergone meniscectomy. 2,3 The presence of meniscal subluxation 4,5 (also called meniscal protrusion 6 or meniscal extrusion, 7 i.e. the protrusion of any part of the meniscus beyond the tibial plateau) 8 may affect proper meniscal function. Spilker and Donzelli modeled the meniscus in various degrees of meniscal subluxation, showing that increasing meniscal subluxation adversely affected load bearing and raised stress concentrations within the meniscus. 9 Kenny reported that radiographic changes known to follow meniscectomy could develop in knees with significant meniscal subluxation of the medial meniscus. 3 He suspected that meniscal subluxation may be related to loss of meniscal function because the radiographic signs are similar to those that follow meniscectomy. Many reports 5,7,10 suggest that meniscal subluxation is a prominent feature of knee OA and contributes to the appearance of joint space narrowing. Magnetic resonance imaging (MRI) is considered to be an ideal modality for evaluating the anatomic structure and position of the meniscus. However, most of the images were obtained with the patients in a supine, non-weight-bearing condition. It is possible that the measured values of meniscal subluxation would be more pronounced if it was assessed under axial loading. With advances in technology, the role of musculoskeletal ultrasonography (MSUS) in orthopedics and rheumatology is evolving rapidly. 11,12 There is growing evidence to show that MSUS is able to provide valuable information about the status of meniscal pathology and at a much lower cost. 13 16 However, to the best of our knowledge, sonographic evaluation of the meniscal position under load has been discussed only sporadically in the literature. 17 Thus, the aim of this study was to describe the sonographic features of meniscal subluxation in the weight-bearing condition, and to determine its relationship with radiographic OA. Materials and Methods Between April 2005 and September 2006, 218 patients with 256 symptomatic knees referred from the outpatient clinic of the orthopedic department were examined successfully with weight-bearing anteroposterior and lateral radiographs and ultrasound images in the routine course of diagnosing reports of problems related to the knee. Eleven knees with medial meniscus tears and seven knees with prior medial meniscus operations were excluded after examination. Two hundred and thirtyeight knees (94 in males, 144 in females) were included in this study. The radiographs were examined for the presence or absence of one or more signs of OA: osteophytes, joint space narrowing, subchondral sclerosis and subchondral cysts. They were graded for overall evidence of OA by two reviewers using the Kellgren-Lawrence Scale: 18 Grade 1, doubtful narrowing of joint space and possible osteophytic lipping; Grade 2, definite osteophytes and possible narrowing of the joint space; Grade 3, definite moderate joint space narrowing (at least 50%), moderate osteophytes, sclerosis or cysts may be present; Grade 4, marked narrowing of the joint space, severe sclerosis, large osteophytes, and cysts may be present. Of the 238 knees, 141 (in 53 males and 88 females) were found to have radiographic signs of OA on weight-bearing anteroposterior radiographs. They were allocated to the OA group. The remaining 97 knees (in 41 males and 56 females) without such signs were placed in the control group. In order to eliminate the age difference between the OA group and controls, knees from the OA group were randomly age-matched to knees from controls. Age differences between pairs of plus or minus 1 year were allowed. Eighty-six knees (43 pairs) were successfully age-matched into an OA group (in 15 males and 28 females) J Formos Med Assoc 2007 Vol 106 No 9 701

C.H. Ko, et al and a control group (in 20 males and 23 females) for additional analysis. Sonographic studies of all the cases were performed by only one examiner (KCH). The patient was placed in a natural standing upright position with knees fully extended and the feet separated enough for good balance. Meniscal subluxation of medial meniscus for each knee was measured by longitudinal ultrasound image at the level of the medial collateral ligament using a 10-MHz linear transducer. The medial collateral ligament (MCL) is a broad flat structure approximately 9 cm long that extends from the medial femoral condyle to the medial aspect of the proximal tibia. It is divided into superficial and deep components that are separated by a zone of loose areolar connective tissue. The superficial component is a band of dense, regular connective tissue anchoring the medial femoral condyle to the proximal tibia. 19 On ultrasound imaging, the MCL appears as a trilaminar structure two hyperechoic bands separated by a thin hypoechoic zone. The hyperreflective bands correspond directly to the superficial and deep ligaments. Loose areolar connective tissue forms the hypoechoic band that separates the superficial and deep components. The medial meniscus is normally hyperechoic and its configuration in a longitudinal view is triangular, with its apex pointing toward the center of the joint. Its base is anchored to the linear hyperechoic deep MCL without any intervening tissue (Figure 1). 19 Strong echoes from the femoral condyle and tibial plateau and the MCL render the meniscus easily identifiable. Meniscal subluxation of medial meniscus was measured as the distance from the outermost edge of the medial meniscus to the border of the tibial plateau (Figure 2). In the presence of marginal osteophytes on the medial border of the tibial plateau, the medial margin of the tibia, instead of the osteophytic projection, was used to measure meniscal subluxation (Figure 3). Once an apparently reliable view was found, the image was frozen; the procedure was then repeated three times, with three different measurements made. To enable measurements to be made on the monitor, a magnified picture was used, to an accuracy of 0.1 mm. For each knee, the mean of these three measurements was used as the final result. The degree of meniscal subluxation in patients with and without radiographic signs of OA from different groups was compared using the Student s MCL FE Femoral condyle Meniscal subluxation TI Tibial plateau Figure 2. Longitudinal sonographic image of the right knee at the level of the medial collateral ligament with a displaced medial meniscus from a 50-year-old woman in the control group. Meniscal subluxation = 2.2 mm. Superficial MCL Deep MCL Meniscus MCL Osteophyte Meniscal subluxation Osteophyte F Femoral condyle T Tibial plateau Femoral condyle F Tibial plateau T Figure 1. Longitudinal sonographic image of the left knee at the level of the medial collateral ligament from a 53- year-old woman. Figure 3. Longitudinal sonographic image of the left knee at the level of the medial collateral ligament with a displaced medial meniscus and osteophytes from a 61-yearold woman in the osteoarthritis group. Meniscal subluxation = 3.8 mm. 702 J Formos Med Assoc 2007 Vol 106 No 9

Meniscal subluxation and osteoarthritis t test. Additional analysis was performed between patients with Kellgren-Lawrence grade 1 2 OA and those with Kellgren-Lawrence grade 3 4 OA on the amount of meniscal subluxation using Student s t test. Results The age and meniscal subluxation data for each group are listed in Table 1. The mean ages of subjects in the OA and control groups were 68.2 ± 10.4 years and 49.5 ± 17.1 years, respectively (p < 0.001). The medial meniscal subluxation for knees in the OA group was 4.3 ± 1.9 mm (range, 2.0 8.9 mm). For knees in the control group, the medial meniscal subluxation was 0.7 ± 0.6 mm (range, 0 2.7 mm). The difference between the two groups was significant (p < 0.001). Meniscal subluxation was present in 51 knees (53%) in the control group. All 141 knees (100%) in the OA group had meniscal subluxation. Subjects in the age-matched study groups ranged in age from 48 to 83 years. Mean ages for the subjects in the age-matched OA and agematched control groups were 63.3 ± 9.6 years and 63.2 ± 9.7 years, respectively. As expected, there was no significant difference in age (p = 0.596). Among the 43 pairs of age-matched knees, only 30% (13 of 43 knees) of the controls had an undisplaced medial meniscus. The medial meniscal subluxation for knees in the age-matched OA group was 4.8 ± 1.7 mm (range, 2.2 8.7 mm). The meniscal subluxation for knees in the age-matched control group was 1.0 ±0.8 mm (range, 0 2.7 mm). The difference between the two groups was significant (p < 0.001). For knees in the OA group, the mean meniscal subluxation was 2.9 ± 0.7 mm (range, 2.0 4.1 mm) in patients with Kellgren- Lawrence grade 1 2 OA and 5.6 ± 1.8 mm (range, 3.7 8.9 mm) in those with Kellgren-Lawrence grade 3 4 OA (Table 2). The difference was significant (p < 0.001). Discussion Knee OA is a common joint disease and is more prevalent in the medial compartment than in the lateral compartment. During normal gait, there is a brief valgus moment after initial contact, and then the knee joint is subjected to an external varus moment throughout stance phase. This varus moment and the subsequent increased loads in the medial compartment are thought to be responsible for the greater incidence of medial compartment osteoarthritis. 20 22 Although the etiology of knee OA can be multifactorial, the importance of the menisci in preventing the evolution of arthritic change is well Table 1. The results of meniscal subluxation for each group* Group n Age (yr) Meniscal subluxation (mm) OA 141 68.2 ± 10.4 (48 86) 4.3 ± 1.9 (2.0 8.9) Controls 97 49.5 ± 17.1 (21 82) 0.7 ± 0.6 (0.0 2.7) Age-matched OA 43 63.3 ± 9.6 (49 83) 4.8 ± 1.7 (2.2 8.7) Age-matched controls 43 63.2 ± 9.7 (48 82) 1.0 ± 0.8 (0.0 2.7) *Data are presented as mean ± standard deviation (range). Table 2. Results of meniscal subluxation for Kellgren-Lawrence grade* Kellgren-Lawrence grade n Meniscal displacement (mm) 1 2 85 2.9 ± 0.7 (2.0 4.1) 3 4 56 5.6 ± 1.8 (3.7 8.9) *Data are presented as mean ± standard deviation (range). J Formos Med Assoc 2007 Vol 106 No 9 703

C.H. Ko, et al understood. The removal of either meniscus may affect the articular surface and eventually lead to destruction of the underlying subchondral bone. The anatomy of the menisci allows for the translation of vertical compressive forces into circumferential (hoop) stresses. The menisci transmit between 50% and 85% of the load across the knee joint in extension and flexion. 23 Clinical and animal studies have shown that preservation of the meniscus improves stability and distributes weight-bearing forces in the knee, thereby reducing degenerating contact stresses across the articular cartilage surfaces. 1,24 28 Increasing meniscal subluxation might affect load transmission and raise stress concentrations within the knee joint (Figure 4). Since meniscal coverage and meniscal height diminishes with subluxation, less coverage and reduced height might increase the risk of cartilage loss. 29 Gale et al measured the amount of meniscal subluxation on conventional MRI of the knee in patients with OA and in asymptomatic volunteers in the coronal plane in the supine position. 5 According to the results of that study, mean meniscal subluxation for the medial and lateral meniscus averaged 5.1 mm and 0.8 mm in OA cases compared with 2.8 mm and 0.2 mm in the control group without OA. Boxheimer et al demonstrated that meniscal protrusion was more frequently present in the medial meniscus and averaged less than 3 mm in normal volunteers in either the sagittal or coronal MR imaging plane. 8 The type and degree of anatomic derangement necessary to permit significant medial meniscal subluxation is uncertain. However, many investigators have reported that degeneration of the meniscus or injuries to the meniscus or its attachments is associated with significant meniscal subluxation. 30 34 Nevertheless, there is still no consensus with respect to the amount of meniscal subluxation that can be considered physiological, but some authors have accepted up to 3 mm of meniscal subluxation as normal. 4,8,35,36 Several studies suggest that meniscal subluxation may be associated with OA. 3,5,7,10,29,33,37 Sugita et al examined the medial menisci and medial tibial plateaus obtained during total knee arthroplasty. 10 There was a mechanical inconsistency between the pattern of preservation of the medial menisci and the location of exposure of subchondral bone on the medial tibial plateaus. They explained that this inconsistency was due to radial displacement (meniscal subluxation) of the medial meniscus preceding narrowing of the medial joint during progression of medial joint Figure 4. (A) Greater contact area and less contact stresses with an undisplaced meniscus. (B) Decreased contact area and increased contact stresses in meniscal subluxation. FEMUR A Undisplaced meniscus Even load distribution on undisplaced meniscus B Meniscal subluxation FEMUR TIBIA TIBIA Increased local stress at the femorotibial articular surfaces in meniscal subluxation 704 J Formos Med Assoc 2007 Vol 106 No 9

Meniscal subluxation and osteoarthritis OA, so that the displaced meniscus was saved from severe degeneration or attrition. Adams et al revealed that meniscal subluxation contributed significantly to radiographic narrowing in patients with early stage OA and articular cartilage loss was a secondary phenomenon following meniscal subluxation in most patients. 7 Gale et al found that meniscal subluxation was highly associated with symptomatic knee OA, and in subjects with OA, increasing meniscal subluxation correlated with the severity of joint space narrowing. 5 Berthiaume et al quantitated the cartilage volume changes in knee OA using MRI, and determined whether meniscal alteration predicted cartilage volume loss over time; they concluded that meniscal tear and extrusion were strongly associated with the progression of symptomatic knee OA. 37 In a more recent study, Hunter et al also demonstrated that meniscal abnormality (whether subluxation or damage) had a major effect on the risk of articular cartilage loss in OA knees. 29 Our study has shown that the measured mean meniscal subluxation for knees with radiographic signs of OA was significantly greater than that for control knees. Meniscal subluxation was observed in 53% (51 of 97 knees) of the controls and in 100% of 141 OA knees. There was no case in whom radiographic signs of OA were observed in the absence of meniscal subluxation. Since significant age differences also existed, the differences in mean meniscal subluxation could have been due merely to the normal aging process, with no true differences between groups. However, when the age-matched groups were compared, the effects of age were eliminated but significant differences in mean meniscal subluxation persisted. Meniscal subluxation was observed in all 43 age-matched OA knees and 70% (30 of 43 knees) of the controls. Furthermore, the greatest meniscal subluxation was seen in knees with advanced radiographic signs of OA. These data suggest that meniscal subluxation is strongly associated with radiographic OA and correlated with the severity of OA changes on plain radiographs. However, meniscal subluxation could be present in non-oa knees as 53% of our controls had a displaced medial meniscus with a maximal subluxation of 2.7 mm. Since the integrity and position of the meniscus are critical for appropriate load bearing and shock absorption, the presence of meniscal subluxation might increase the peak and average contact stresses at the femorotibial articular surfaces and accelerate the development of OA. 29,33,37 Given the potency of meniscal subluxation as a risk factor, our study found that MSUS is able to identify this problem early before the significant OA changes seen on conventional radiographs. Pomeranz has described the phenomenon of meniscal pseudosubluxation in which the meniscus appears to be subluxed. 38 This is most commonly seen with loose meniscocapsular attachments, particularly in degenerative joint disease. The criterion for this condition is the meniscus being located outside the edge, drawn from the most superficial aspect of the femoral condyle. In most of these cases, the position of the meniscus on the tibial plateau is stable, and a true lateral displacement does not occur. So, it is an important issue that the edge of the tibia must be used as the reference point for the proper determination of meniscal position and subluxation because the attachment of the meniscus to the tibia is much stronger than its attachment to the femur. 4 With regard to meniscal position and pathology, most of the information is derived from MRI for visualization of the knee menisci. 3,5 7,29,31,33,37 However, in most instances, MRI study is unable to visualize the meniscus under axial loading since the patient is scanned in the supine position. Within the last decade, MSUS has become an established imaging technique for the diagnosis and follow-up of patients with musculoskeletal disorders. Many authors suggest that MSUS is an acceptable imaging alternative for visualization of meniscal pathology. 13 17 Other advantages of MSUS include its noninvasiveness, portability, relatively low cost, lack of ionizing radiation, and its ability to be repeated as often as necessary. Although there is little evidence with respect to the use of MSUS in the evaluation of medial meniscal subluxation under load, our results are comparable to previously published data obtained J Formos Med Assoc 2007 Vol 106 No 9 705

C.H. Ko, et al by MRI. Boxheimer et al carried out MRI studies on 22 asymptomatic patients in a weight-bearing position and found that medial meniscal subluxation was present in 50% (11 of 22) of the cases, and the maximum subluxation in the coronal plane was 2.6 mm. 8 These results are similar to those obtained in our study, where we found 53% (51 of 97) of knees in our control group having displaced menisci with the greatest subluxation of 2.7 mm. Furthermore, Boxheimer et al also showed that the measured meniscal subluxation might have been more frequent if it was observed under weight-bearing conditions. 8 This fact was observed in their study, where greater frequency of meniscal subluxation was obtained in the standing weight-bearing knees (11 of 22) than in the supine non-weight-bearing knees (9 of 22). Since medial compartment OA is more prevalent in people with OA 20 22 and the MCL can be easily identified by MSUS with good reproducibility, our study was designed to measure meniscal subluxation of the medial meniscus at the level of the MCL. We believe that measuring subluxation at the level of the MCL alone may overlook meniscal displacement in other directions, and similar subluxations may occur in the lateral meniscus but were not estimated in this study. The limitations of this study include: (1) the threshold of pathologic subluxation of the meniscus was not identified; (2) factors that lead to meniscal subluxation were not studied; (3) as indicated above, we only examined meniscal subluxation at the level of the MCL, thus, meniscal displacement in other directions were not estimated. In conclusion, we employed MSUS to assess the meniscal position of the knee under weightbearing condition so that the dynamic contribution of loading to the meniscal subluxation was observed. The results showed that meniscal subluxation is a prominent feature of knee OA and could be considered a risk factor for the development of OA. However, meniscal subluxation could also be present in people without radiographic OA, but it is in general < 3 mm. MSUS is able to detect this occult meniscal derangement early before the appearance of radiographic OA changes. References 1. Fairbank TJ. Knee joint changes after meniscectomy. J Bone Joint Surg Br 1948;30:664 70. 2. Ahlback S. Osteoarthritis of the knee: a radiographic investigation. Acta Radiol 1968;277(Suppl):3 72. 3. Kenny C. Radial displacement of the medial meniscus and Fairbank s signs. Clin Orthop 1997;339:163 73. 4. Breitenseher MJ, Trattnig S, Dobrocky I, et al. MR imaging of meniscal subluxation in the knee. Acta Radiol 1997;38: 876 9. 5. Gale DR, Chaisson CE, Totterman SM, et al. Meniscal subluxation: association with osteoarthritis and joint space narrowing. Osteoarthritis Cartilage 1999;7:526 32. 6. Miller TT, Staron RB, Feldman F, et al. Meniscal position on routine MR imaging of the knee. Skeletal Radiol 1997;26:424 7. 7. Adams JG, McAlindon T, Dimasi M, et al. Contribution of meniscal extrusion and cartilage loss to joint space narrowing in osteoarthritis. Clin Radiol 1999;54:502 6. 8. Boxheimer L, Lutz AM, Treiber K, et al. MR imaging of the knee: position related changes of the menisci in asymptomatic volunteers. Invest Radiol 2004;39:254 63. 9. Spilker RL, Donzelli PS. A basic finite element model of the meniscus for stress strain analysis. In: Mow VC, Arnoczky SP, Jackson DW, eds. Knee Meniscus: Basic and Clinical Foundations. New York: Raven Press, 1992: 91 115. 10. Sugita T, Kawamata T, Ohnuma M, et al. Radial displacement of the medial meniscus in varus osteoarthritis of the knee. Clin Orthop 2001;387:171 7. 11. D Agostino MA, Breban M. Ultrasonography in inflammatory joint disease: why should rheumatologists pay attention? Joint Bone Spine 2002;69:252 5. 12. Grassi W, Cervini C. Ultrasonography in rheumatology: an evolving technique. Ann Rheum Dis 1998;57:268 71. 13. Selby B, Richardson ML, Nelson BD, et al. Sonography in the detection of meniscal injuries of the knee: evaluation in cadavers. AJR Am J Roentgenol 1987;149:549 53. 14. Casser HR, Sohn C, Kiekenbeck A. Current evaluation of sonography of the meniscus. Results of a comparative study of sonographic and arthroscopic findings. Arch Orthop Trauma Surg 1990;109:150 4. 15. Court-Payen M. Sonography of the knee: intra-articular pathology. J Clin Ultrasound 2004;32:481 90. 16. Najafi J, Bagheri S, Lahiji FA. The value of sonography with micro convex probes in diagnosing meniscal tears compared with arthroscopy. J Ultrasound Med 2006;25: 593 7. 17. Verdonk P, Depaepe Y, Desmyter S, et al. Normal and transplanted lateral knee menisci: evaluation of extrusion using magnetic resonance imaging and ultrasound. Knee Surg Sports Traumatol Arthrosc 2004;12:411 9. 18. Kellgren JH, Lawrence JS. Radiological assessment of osteoarthritis. Am Rheum Dis 1947;16:494 501. 706 J Formos Med Assoc 2007 Vol 106 No 9

Meniscal subluxation and osteoarthritis 19. Van Holsbeeck M, Introcaso J. Musculoskeletal Ultrasound, 2 nd edition. St. Louis, MO: Mosby, 2001:172 3, 588. 20. Andriacchi TP. Dynamics of knee malalignment. Orthop Clin North Am 1994;25:395 403. 21. Kaufman KR, Hughes C, Morrey BF, et al. Gait characteristics of patients with knee osteoarthritis. J Biomech 2001; 34:907 15. 22. Sharma L, Hurwitz DE, Thonar EJMA, et al. Knee adduction moment, serum hyaluronan level, and disease severity in medial tibiofemoral osteoarthritis. Arthritis Rheum 1998;41: 1223 40. 23. Insall JN, Scott WN. Surgery of the Knee Volume 1, 3 rd edition. Oxford: Churchill Livingstone, 2001:477. 24. Hede A, Larsen E, Sandberg H. Partial versus total meniscectomy. A prospective, randomised study with long-term follow-up. J Bone Joint Surg Br 1992;74:118 21. 25. Jackson JP. Degenerative changes in the knee after meniscectomy. Br Med J 1968;2:525 7. 26. Johnson RJ, Kettelkamp DB, Clark W, et al. Factors affecting late results after meniscectomy. J Bone Joint Surg Am 1974;56:719 29. 27. Milachowski KA, Weismeier K, Wirth CJ. Homologous meniscus transplantation. Experimental and clinical results. Int Orthop 1989;13:1 11. 28. Veth RP. Clinical significance of knee joint changes after meniscectomy. Clin Orthop 1985;198:56 60. 29. Hunter DJ, Zhang YQ, Niu JB, et al. The association of meniscal pathologic changes with cartilage loss in symptomatic knee osteoarthritis. Arthritis Rheum 2006;54: 795 801. 30. Clancy WG, Keene JS, Golenz TH. Symptomatic dislocation of the anterior horn of the medial meniscus. Am J Sports Med 1984;12:57 64. 31. Costa CR, Morrison WB, Carrino JA. Medial meniscus extrusion on knee MRI: is extent associated with severity of degeneration or type of tear? AJR Am J Roentgenol 2004;183:17 23. 32. Helfet AJ. Mechanisms of derangement of medial semilunar cartilage and their management. J Bone Joint Surg 1959;41B:319 36. 33. Lerer DB, Umans HR, Hu MX. The role of meniscal root pathology and radial meniscal tear in medial meniscal extrusion. Skeletal Radiol 2004;33:569 74. 34. Pagnani MJ, Cooper DE, Warrren RF. Extrusion of the medial meniscus. Arthroscopy 1991;7:297 300. 35. Yang A, Yang SS, Greif WL, et al. Role of meniscal subluxation in degenerative arthritis of the knee. Demonstration with coronal MR imaging and the Oreo cookie model. Radiology 1993;189:380. 36. Puig L, Monllau JC, Corrales M, et al. Factors affecting meniscal extrusion: correlation with MRI, clinical, and arthroscopic findings. Knee Surg Sports Traumatol Arthrosc 2006;14:394 8. 37. Berthiaume MJ, Raynauld JP, Martel-Pelletier J, et al. Meniscal tear and extrusion are strongly associated with progression of symptomatic knee osteoarthritis as assessed by quantitative magnetic resonance imaging. Ann Rheum Dis 2005;64:556 63. 38. Pomeranz SJ. Gamuts and Pearls in MRI. Ohio: MRI & EFI Publications, 1993:406. J Formos Med Assoc 2007 Vol 106 No 9 707