Changes in Magnetic Resonance Imaging Signal Intensity of Transplanted Meniscus Allografts Are Not Associated With Clinical Outcomes

Changes in Magnetic Resonance Imaging Signal Intensity of Transplanted Meniscus Allografts Are Not Associated With Clinical Outcomes Dae-Hee Lee, M.D., Bum-Sik Lee, M.D., Jong-Won Chung, M.D., Jong-Min Kim, M.D., Kyung-Sook Yang, Ph.D., Eun-Jong Cha, Ph.D., and Seong-Il Bin, M.D.

Purpose: To evaluate changes in intrameniscal signal intensity (IMSI) of transplanted allografts during the first year after meniscus allograft transplantation (MAT) by use of serial magnetic resonance imaging, as well as to analyze the relation between IMSI and clinical outcome. Methods: This prospective study involved 43 patients who underwent MAT between 2006 and 2007 after diagnosis of total or subtotal meniscectomized knees. The mean patient age at the time of surgery was 35.8 years (range, 17 to 46 years). Allografts were assessed by conventional magnetic resonance imaging performed at 6 weeks and 3, 6, and 12 months after MAT. The ratio of the signal intensity of the transplanted meniscus allograft to that of the control normal meniscus in the ipsilateral knee was calculated to obtain a standardized signal intensity value. IMSI was assessed in terms of postoperative time and location (anterior v posterior horn). The Lysholm score was used to evaluate knee function. Results: The IMSI of transplanted allograft menisci was higher than that for nontransplanted menisci at all 4 postoperative time points (P ⬍ .01). The anterior horn allograft IMSI was greater than the posterior horn allograft IMSI at all time points (P ⬍ .01). The allograft IMSI increased starting 3 months postoperatively for the anterior horn (F3,40 ⫽ 7.5, P ⬍ .01) and 6 months postoperatively for the posterior horn (F3,40 ⫽ 9.2, P ⬍ .01). These increases were maintained to the final assessment at 1 year postoperatively. No correlation was found between IMSI and postoperative Lysholm score. Conclusions: Transplanted allograft menisci had higher signal intensities than normal menisci. Signal intensity was higher for the anterior horn than the posterior horn throughout the first postoperative year. Signal intensity increased over time, and this increase was maintained at 1 year postoperatively. However, signal intensity was not related to clinical outcome. Level of Evidence: Level II, development of diagnostic criteria based on analysis of consecutive patients, applying a universally recognized gold standard.

S

From the Department of Orthopaedic Surgery, College of Medicine, Korea University, Anam Hospital (D-H.L.), Seoul; Department of Biostatistics, College of Medicine, Korea University (K-S.Y.), Seoul; Biomedical Engineering Department, School of Medicine, Chungbuk National University (E-J.C.), Cheongju; and Department of Orthopedic Surgery, College of Medicine, University of Ulsan, Asan Medical Center (B-S.L., J-W.C., J-M.K., S-I.B.), Seoul, South Korea. Supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2009-0063258). The authors report no conflict of interest. Received May 18, 2010; accepted March 25, 2011. Address correspondence to Seong-Il Bin, M.D., Department of Orthopedic Surgery, College of Medicine, University of Ulsan, Asan Medical Center, 388-1, Poongnap-2dong, Songpa-gu, Seoul, 138-736, South Korea. E-mail: [email protected] © 2011 by the Arthroscopy Association of North America 0749-8063/10306/$36.00 doi:10.1016/j.arthro.2011.03.087

hort-,1,2 medium-,3,4 and long-term5-7 clinical studies have indicated that meniscus allograft transplantation (MAT) is a useful therapeutic strategy for symptomatic subtotal/total meniscectomized knees in young patients. Accurate postoperative allograft evaluation is important for determining the timeline for patients to resume various activities, as well as to determine whether the procedure has been successful. However, such postoperative evaluation is usually based on clinical assessments, which can have limited accuracy.8 Magnetic resonance imaging (MRI) is a good tool for evaluating primary meniscal pathology.9 However, the accuracy of MRI for evaluating postoperative meniscal status (i.e., meniscal repair or partial meniscectomy) is lower than that for presurgery evaluation.10 Such inaccuracy can predispose some orthopaedic surgeons to perform diagnostic “second-look” arthroscopy or direct magnetic resonance arthrography for

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postoperative evaluation. However, those procedures are invasive and carry a greater risk of complications than MRI. A greater understanding of the characteristics of MRI findings after MAT is likely to increase the accuracy of MRI graft assessment. Whereas MAT grafts usually show increased signal intensity on MRI,11-13 the clinical significance of this increased intensity has not been established. In our practice we have observed that postoperative MRI shows an increase in signal intensity in the transplanted allograft in nearly all MAT patients, and that this increase predominantly occurs in anterior horn grafts compared with posterior horn grafts. The difficulty for clinicians is to determine whether such a signal increase is due to an allograft tear, especially in cases where the physical examination findings are inconclusive. If an increase is due to a tear, further surgery is likely to be performed, but if it is not due to a tear, continued rehabilitation is likely to be recommended. The kinetics of the increase, as well as the length of time for which it persists, are also yet to be established. To date, no study has used serial MRI to examine changes in allograft signal intensity after MAT. The purpose of this study was to investigate the effect of postoperative time and graft location on intrameniscal signal intensity (IMSI) after MAT. The study used MRI data taken at 4 time points over the first postoperative year. The study also examined whether there was any relation between IMSI and clinical outcome. We hypothesized that (1) IMSI differed according to meniscus allograft location, (2) IMSI changed during the postoperative healing process, and (3) IMSI after MAT does not correlate with clinical outcome. METHODS Study Design and Clinical Evaluation This study was a prospective longitudinal trial designed to analyze changes in transplanted allograft MRI signal intensity after MAT. All patients who were candidates for MAT at our institution from 2006 and 2007 were considered for study inclusion. The indication for meniscus transplant included 1-compartment pain attributed to a lack of a meniscus, articular chondral wear of Outerbridge grade III (focalized IV) or less confirmed by previous arthroscopic surgery, and normal mechanical alignment, which was assessed both clinically and with long-leg standing films. Con-

traindications were uncorrected axis deformity involving greater than 3° of deviation toward the involved compartment, generalized tricompartmental arthritis, or uncorrected joint instability caused by ligament structure deficiency. The study was explained to each patient, and each patient provided written informed consent. The study protocol was approved by our institutional review board (AN10010-001). A priori power analysis was performed to determine the sample size by use of the 2-sided hypothesis test at an ␣ level of .05 and a power of 0.8. The results of a pilot study involving 8 cases indicated that 37 knees were required to detect a significant difference in signal intensity between transplanted allografts and nontransplanted menisci on the ipsilateral side, which was the primary outcome measure in the present study. This study finally involved 43 patients, indicating adequate power (0.874) for detecting a significant difference between the 2 horns. Surgical Technique and Postoperative Rehabilitation All menisci in the medial compartment were transplanted by the modified double– bone plug technique described by Shelton and Dukes,14 and all menisci in the lateral compartment were transplanted by the modified key hole technique described by Wilcox and Goble.15 Immediately after surgery, patients began quadriceps sets, straight-leg raises, and calf pumps. At 1 or 2 days after surgery, the Hemovac drain was removed, and continuous passive motion exercises were commenced during the first postoperative week. The goal for range-of-motion exercise was to achieve full extension equal to the contralateral side within 1 week, 90° of flexion within 4 weeks, and 120° of flexion at 6 to 8 weeks. The patients were allowed only toetouch weight bearing during the first 2 weeks postoperatively, which was slowly increased to 50% of body weight at the fourth week and to full weight bearing at the sixth week. Rehabilitation continued for 3 months and focused on restoring the full range of motion and quadriceps strength. MRI Evaluation Conventional MRI by use of a 1.5-T magnet was used to evaluate the meniscus allograft status at 4 post-MAT time points: 6 weeks, and 3, 6, and 12 months. We obtained sagittal T2-weighted and proton density–weighted (PDW) fast spin echo and T1weighted spin echo images, with coronal PDW fast spin echo images. The MRI parameters were as fol-

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FIGURE 1. MRI scan showing region-of-interest method used to determine mean signal intensity for transplanted allografts. To optimize measurement reliability, we calculated pixel values in each triangle automatically formed by the connection of each of the 3 meniscal vertices. (Min, minimum; Max, maximum; StdDev, standard deviation.)

lows: repetition time (TR)/echo time (TE) of 2,760 milliseconds/15 milliseconds for PDW images, TR/TE of 3,900 milliseconds/105 milliseconds for T2-weighted images, TR/TE of 530 milliseconds/12 milliseconds for T1-weighted images, and TR/TE/flip angle of 700 milliseconds/18 milliseconds/20° for gradient echo images; 1 echo train length; 3-mm slice thickness; 0.6- to 0.9-mm intervals; 512 ⫻ 512 matrix; scan time of 4 minutes 18 seconds to 4 minutes 23 seconds; and 16-cm field of view. After assessment of all sagittal slices of T1- and T2-weighted images for each knee, measurements were taken from midsagittal images on both the transplanted and nontransplanted sides. The anterior and posterior horns of the allograft were compared with findings from the normal meniscus anterior and posterior horns on the ipsilateral side. IMSI of the transplanted graft was measured by use of a free line region-of-interest tool in a picture archiving and communication system (PI View STAR, version 5025; Infinitt, Seoul, South Korea). The region-of-interest area was first marked by taking arbitrary fixed points bordering the maximum area of the anterior and posterior horns of the allograft. The property was then defined by delineating the size and color level by

using the specialized software in the picture archiving and communication system. The color level indicating the mean value of signal intensity was obtained from the screen. Thus the mean signal intensity value for the maximum area at a particular cut was used for analysis (Fig 1). Similar procedures were performed for both the transplanted and normal nontransplanted menisci. IMSI can appear to vary between examinations because of inconsistency in the degree of screen contrast. Therefore we decided to use the ratio of signal intensity of the transplanted meniscus allograft to that of the control normal meniscus for standardization of the signal intensity. Serial MRI studies were independently evaluated by 2 orthopaedic surgeons with significant experience in the field. Each surgeon twice measured IMSI for all 43 cases, with an interval of 2 weeks between measurements. The mean of the measurements taken by the 2 surgeons was used for analysis. The reliability of IMSI measurements was assessed by use of the intraclass correlation coefficient, which quantifies what proportion of the difference is due to measurement variability. In general, the intraclass correlation coefficient is interpreted similar to ␬. Hence, intraclass correlation coefficient values greater

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than 0.75 represent fair to good agreement, whereas values lower than 0.40 represent poor agreement. Data Analysis Statistical analysis was performed with SPSS software, version 12 (SPSS, Chicago, IL). Signal intensities between the anterior or posterior horn of the transplanted allograft and the nontransplanted normal side were compared by use of a 1-sample t test. P ⬍ .025 indicated that the parameter mean of the variable was larger than 1. The ratio of signal intensity of the transplanted meniscus allograft to that of the control normal meniscus in the anterior and posterior horns was compared by use of paired Student t tests because the values were normally distributed. Paired Student t tests were also used to compare preoperative and postoperative Lysholm scores. P ⬍ .05 was considered to indicate a significant difference. Repeated-measures analysis of variance (ANOVA) was used to evaluate changes in IMSI over time. This method takes into account outcome measurements taken over time, and successive outcomes were correlated. A trend analysis (in terms of time) was performed by use of the polynomial contrasts function of repeated-measures ANOVA. The multiple comparisons method was used when variations were identified as statistically significant. The Bonferroni multiple comparisons method was used to adjust the overall level of significance when the effect of a variable was identified as statistically significant. The correlation coefficient between the IMSI on MRI at each time point and the postoperative Lysholm score at the last follow-up was assessed by use of Pearson correlation analysis because all variables were continuous and parametric. When P ⬍ .05, correlation coefficients (r) were calculated (ranging from –1 to ⫹1) to determine positive or negative correlations. RESULTS Demographic Data and Clinical Outcomes Of the 52 patients (52 knees) initially approached for the study, 49 patients agreed to take part. After eligibility assessments, 47 patients were enrolled. The final analysis involved data from 43 patients (4 patients were excluded because MRI data from only 3 rather than 4 postoperative time points were available). No patients were lost to follow-up. At the last follow-up (conducted at a mean postoperative time of 2.7 years; range, 2.3 to 3.7 years), all patients personally evaluated their knee function using the Lysholm

knee score (maximum score, 100 points). In this system, 95 to 100 points reflects an excellent outcome; 84 to 94, a good outcome; 65 to 83, a fair outcome; and less than 65, a poor outcome. However, no MRI follow-up was conducted. Therefore all IMSI data were obtained in the first year after MAT. The mean age of the 43 patients (34 male and 9 female patients) was 35.8 years (SD, 8.7 years). There were 6 medial and 37 lateral transplants. The mean Lysholm score for all patients increased from 72 (range, 62 to 84) preoperatively to 88 (range, 78 to 96) at 2 years postoperatively (P ⬍ .01). The final follow-up Lysholm knee scores indicated that 16 patients (37%) had excellent outcomes, 25 (58%) had good outcomes, and 2 (5%) had fair outcomes. IMSI on Serial MRI and Correlation With Clinical Outcome The IMSI values were expressed as a ratio of the intensity between the transplanted meniscus allograft and that of the control normal meniscus. Regarding the reliability of IMSI measurements, interobserver reliability ranged from 0.714 to 0.823 and intraobserver reliability ranged from 0.731 to 0.853 on serial MRI at the 4 time points, indicating good reliability. The mean IMSI values for allografts of the posterior and anterior horns at the 4 time points are shown in Table 1. The allograft mean IMSI value was greater than 1 at all times for either horn on both T1- and T2-weighted images (P ⬍ .001 for all comparisons). This indicates that the allograft meniscus signal intensity was higher than the nontransplanted ipsilateral meniscus. In addition, the IMSI value for the anterior horn was always higher than the value for the posterior horn on all T1-weighted images (Fig 2A) and T2-weighted images (Fig 2B) and at all times. The mean values derived from the T1- and T2weighted images of each horn did not significantly differ between the 4 different times (P ⬎ .05), and hence we used the mean values from T1- and T2weighted images to evaluate the effect of time on signal intensity. For the anterior horn, the mean IMSI values were 2.11, 3.03, 3.14, and 3.03 at 6 weeks, 3 months, 6 months, and 12 months, respectively. For the posterior horn, the values were 1.28, 1.56, 1.62, and 1.93, respectively. Statistical analysis showed that these mean signal intensity values differed at each time point for both the anterior horn (F3,40 ⫽ 7.5, P ⬍ .01) and posterior horn (F3,40 ⫽ 9.2, P ⬍ .01) (Fig 3). For the anterior horn, the mean IMSI values were higher at 3 months than at 6 weeks and remained

SIGNAL INTENSITY OF MENISCUS ALLOGRAFTS TABLE 1. Mean MRI Signal Intensity Ratios for Anterior and Posterior Horn Allografts at 4 Postoperative Times Postoperative Time 6 wk AH on T1 AH on T2 PH on T1 PH on T2 3 mo AH on T1 AH on T2 PH on T1 PH on T2 6 mo AH on T1 AH on T2 PH on T1 PH on T2 1 yr AH on T1 AH on T2 PH on T1 PH on T2

Mean Ratio Value

Standard Error

P Value

2.08 2.14 1.29 1.27

0.15 0.14 0.06 0.11

⬍ .001 ⬍ .001 ⬍ .001 .018

3.06 3.00 1.58 1.54

0.38 0.30 0.12 0.12

⬍ .001 ⬍ .001 ⬍ .001 ⬍ .001

3.63 2.66 1.80 1.44

0.42 0.23 0.13 0.70

⬍ .001 ⬍ .001 ⬍ .001 ⬍ .001

3.42 2.64 2.12 1.74

0.30 0.22 0.19 0.12

⬍ .001 ⬍ .001 ⬍ .001 ⬍ .001

NOTE. The mean signal intensity ratio represents a ratio of signal intensity of the transplanted meniscus allograft to that of the control normal meniscus. P ⬍ .025 indicated that the parameter mean of the variable was larger than 1. Abbreviations: AH, anterior horn; PH, posterior horn; T1, T1weighted image; T2, T2-weighted image.

higher over the year (Table 2). For the posterior horn, the mean values were higher at 6 months compared with 6 weeks and 3 months and remained higher through 1 year (Table 3). Therefore we performed a trend analysis over time, using the polynomial contrasts available in the repeated-measures ANOVA subprogram. The use of such contrasts indicated that a significant linear effect was evident; the mean increases over time in terms of both anterior horn MAT (F1,42 ⫽ 17.15, P ⬍ .001) and posterior horn MAT (F1,42 ⫽ 26.44, P ⬍ .001). In terms of anterior horn MAT, both the linear and quadratic trends were significant (F1,42 ⫽ 6.28, P ⫽ .016 for quadratic component). These findings indicated that IMSI increased linearly over time in terms of the anterior horn and that a quadratic trend was also in play, as reflected by attainment of a plateau on final measurement. However, no significant posterior horn quadratic trend was evident (F1,42 ⫽ 0.02, P ⫽ .901). We used correlation analysis to determine whether there was any association between IMSI and the Lysholm score at 2 years postoperatively. This analysis showed that there was no such association at either

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horn, at any time point, on either T1- or T2-weighted images (Table 4). The medial meniscus group was small (n ⫽ 6), so it was not possible to compare IMSI and Lysholm scores between this group and the lateral meniscus group. DISCUSSION This study analyzed changes in transplanted allograft IMSI after MAT by use of serial MRI studies at 4 different times over the first postoperative year. Standardized values were created by use of a ratio between the allograft and nontransplanted meniscus intensities. The study found that transplanted allograft menisci had a higher signal intensity over the first postoperative year compared with nontransplanted ipsilateral menisci. We also found that anterior horn allograft intensity was greater than posterior horn allograft intensity. Previous studies have incidentally reported that transplanted grafts show an increase in signal intensity on post-MAT MRI. Using the criteria of Stoller et al.,16 2 recent studies reported increased signal inten-

FIGURE 2. Comparison of signal intensities between anterior horn (AH) and posterior horn (PH) at 4 postoperative time points according to T1-weighted (A) and T2-weighted (B) images. The IMSI value for the AH was always higher than that for the PH on all T1- and T2-weighted images.

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FIGURE 3. Change in IMSI on 4 serial MRI studies (T2weighted images) after lateral MAT. It should be noted that the value for the anterior horn was always higher than the value for the posterior horn at 4 different time points (6 weeks postoperatively [A], 3 months postoperatively [B], 6 months postoperatively [C], and 1 year postoperatively [D]). The rises in signal intensity evident after 3 months (anterior horn) and 6 months (posterior horn) persisted for 12 months after MAT.

sity after MAT.4,17 One of those studies analyzed 28 meniscus transplants using MRI with a 0.7-T magnet at a mean of 35 months after implantation.4 All but 1 transplant showed increased IMSI, which was rated as grade 1 in 13 transplants, grade 2 in 11, and grade 3 in 3 (1 transplant could not be evaluated). The other study reported that all 8 cryopreserved meniscal transplants showed increased signal intensity on postoperative MRI at a mean of 2 years.17 Intrameniscal inTABLE 2.

tensity was rated as grade 1 in 2 cases, grade 2 in 4, and grade 3 in 2. However, there is no general agreement regarding the time to development, or persistence, of increased IMSI. We found that the signal intensity was higher in transplanted allografts than in ipsilateral nontransplanted normal menisci over the first postoperative year. In addition, we observed increased signal intensity within allografts from 3 months (anterior horn)

Differences Between Anterior Horn MRI IMSI Values at 4 Postoperative Time Points 95% Confidence Interval

Postoperative Time MRI MRI MRI MRI MRI MRI MRI MRI MRI MRI MRI MRI

at at at at at at at at at at at at

6 6 6 3 3 3 6 6 6 1 1 1

wk v MRI at 3 mo wk v MRI at 6 mo wk v MRI at 1 yr mo v MRI at 6 wk mo v MRI at 6 mo mo v MRI at 1 yr mo v MRI at 6 wk mo v MRI at 3 mo mo v MRI at 1 yr yr v MRI at 6 wk yr v MRI at 3 mo yr v MRI at 6 mo

Mean Difference

Standard Error

Lower

Upper

P Value

–0.92 –1.03 –0.92 0.92 –0.11 0.00 1.03 0.11 0.11 0.92 0.00 –0.11

0.27 0.24 0.20 0.27 0.25 0.29 0.24 0.25 0.23 0.20 0.29 0.23

–1.66 –1.71 –1.47 0.17 –0.80 –0.81 0.35 –0.58 –0.52 0.37 –0.81 –0.74

–0.17 –0.35 –0.37 1.66 0.58 0.81 1.71 0.80 0.74 1.47 0.81 0.52

.008 .001 ⬍ .001 .008 ⬎ .999 ⬎ .999 .001 ⬎ .999 ⬎ .999 ⬍ .001 ⬎ .999 ⬎ .999

SIGNAL INTENSITY OF MENISCUS ALLOGRAFTS TABLE 3.

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Differences Between Posterior Horn MRI IMSI Values at 4 Postoperative Time Points 95% Confidence Interval

Postoperative Time MRI MRI MRI MRI MRI MRI MRI MRI MRI MRI MRI MRI

at at at at at at at at at at at at

6 6 6 3 3 3 6 6 6 1 1 1

wk v MRI at 3 mo wk v MRI at 6 mo wk v MRI at 1 yr mo v MRI at 6 wk mo v MRI at 6 mo mo v MRI at 1 yr mo v MRI at 6 wk mo v MRI at 3 mo mo v MRI at 1 yr yr v MRI at 6 wk yr v MRI at 3 mo yr v MRI at 6 mo

Mean Difference

Standard Error

Lower

Upper

P Value

–0.28 –0.34 –0.65 0.28 –0.06 –0.37 0.34 0.06 –0.31 0.65 0.37 0.31

0.11 0.10 0.13 0.11 0.07 0.16 0.10 0.07 0.15 0.13 0.16 0.15

–0.59 –0.62 –0.99 –0.03 –0.26 –0.81 0.06 –0.14 –0.72 0.30 –0.08 –0.11

0.03 –0.06 –0.30 0.59 0.14 0.08 0.62 0.26 0.11 0.99 0.81 0.72

.090 .012 ⬍ .001 .090 ⬎ .999 .162 .012 ⬎ .999 .280 ⬍ .001 .162 .280

and 6 months (posterior horn), and we found that those increases were maintained out to 1 year. Persistent increased signal intensity was also reported by other authors using post–meniscal repair MRI.18 The changes in signal intensity at 3 months (anterior horn) or 6 months (posterior horn) after MAT in this study are consistent with the time frame of meniscal TABLE 4. Assessment of Correlation Between Transplanted Allograft Signal Intensity and Postoperative Lysholm Score (2 Years) Postoperative Time 6 wk AH on T1 AH on T2 PH on T1 PH on T2 3 mo AH on T1 AH on T2 PH on T1 PH on T2 6 mo AH on T1 AH on T2 PH on T1 PH on T2 1 yr AH on T1 AH on T2 PH on T1 PH on T2

Pearson Correlation Coefficient (r)

P Value

0.247 0.194 –0.021 –0.166

.110 .212 .893 .289

0.020 0.102 –0.059 0.086

.897 .516 .709 .584

–0.033 –0.058 –0.046 –0.155

.834 .711 .771 .322

–0.066 0.295 –0.052 0.042

.675 .055 .739 .790

Abbreviations: AH, anterior horn; PH, posterior horn; T1, T1weighted image; T2, T2-weighted image.

tear healing and repopulation after MAT reported in previous studies. The process of meniscal healing is known as the metaplasia of initial fibrovascular scar tissue into fibrocartilage. In an experimental animal study of meniscal healing, Arnoczky et al.19 showed that the increase in IMSI was observed in the repair tissue at 6 months and speculated that meniscal healing can take up to 6 months, at which time the repair tissue was still histologically different from normal meniscal tissue. Increased intrameniscal intensity after MAT may also be a result of the repopulation process in which allograft donor cells are replaced by host cells. A dog study investigated the morphology and metabolic activity of 14 transplanted cryopreserved meniscus allografts over a period of 6 months.20 Repopulation was found to commence from 2 weeks after MAT by cells that appeared to originate from the peripheral soft-tissue (synovial) attachments. At 3 and 6 months after MAT, cells resembling fibrochondrocytes had repopulated the entire allograft except for the central deep portion. However, the collagen architecture of the allografts changed to a loss of normal collagen orientation in the superficial and subsuperficial layers of the meniscus. Thus remodeling and poor organization of collagen fibers in the repopulated meniscus allograft may underlie the persistent increased signal intensity throughout the first postoperative year observed in our study. Verdonk et al.21 examined 25 meniscus allografts within the first postoperative year using MRI. Of these patients, 17 also underwent MRI at a mean of 12 years postoperatively. The 12-year data showed that the allograft signal intensity was grade III in 10 cases

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(59%), and of these 10 patients, 7 also had grade III signal intensity within the first postoperative year. These findings indicate that increased allograft signal intensity can persist for longer than many investigators would expect and can perhaps even be permanent. Noyes et al.4 also showed that replacement with more randomized and disorganized collagen tissue could cause increased signal intensity after implantation. We believe that the persistently increased intrameniscal intensity that we observed indicated normal healing rather than degenerative changes or tears. Consistent with this, we found no correlation between IMSI and Lysholm scores at 2 years postoperatively. This study found that intrameniscal intensity was always higher in anterior horn grafts compared with posterior horn grafts. In cases involving vague knee joint pain after MAT, it is important to distinguish between an allograft tear and IMSI because the former may indicate further surgery whereas the latter may merely require further rehabilitation. We cannot rule out the possibility that a meniscal tear caused IMSI because second-look arthroscopy was not performed after MAT was conducted. However, the knowledge that signal intensity is greater at the anterior horn after MAT may help prevent unnecessary surgery in some cases where an anterior allograft horn tear is suspected but the physical examination is inconclusive. One limitation of this study was the relatively small number of patients. However, the study was strengthened by its prospective design and by the use of strict inclusion and exclusion criteria, which meant that cases with only 3 rather than 4 postoperative MRI studies were excluded. In addition, the sample size had acceptable statistical power for the endpoint (i.e., detecting a significant difference in signal intensity between transplanted allografts and nontransplanted menisci on the ipsilateral side). Another study limitation was the absence of an immediate postoperative MRI study to use as a baseline for the IMSI. However, immediate, uniform low signal intensity on MRI within 12 hours after MAT has been reported by Potter et al.11 CONCLUSIONS Transplanted allograft menisci had higher signal intensities than normal menisci. Signal intensity was higher for the anterior horn than the posterior horn throughout the first postoperative year. Signal intensity increased over time, and this increase was maintained at 1 year postoperatively. However, signal intensity was not related to clinical outcome.

REFERENCES 1. Stollsteimer GT, Shelton WR, Dukes A, Bomboy AL. Meniscal allograft transplantation: A 1- to 5-year follow-up of 22 patients. Arthroscopy 2000;16:343-347. 2. Ryu RK, Dunbar V, WH, Morse GG. Meniscal allograft replacement: A 1-year to 6-year experience. Arthroscopy 2002;18:989-994. 3. Cole B, Dennis M, Lee S, et al. Prospective evaluation of allograft meniscus transplantation: A minimum 2-year followup. Am J Sports Med 2006;34:919-927. 4. Noyes F, Barber-Westin S, Rankin M. Meniscal transplantation in symptomatic patients less than fifty years old. J Bone Joint Surg Am 2004;86:1392-1404. 5. Graf KW Jr, Sekiya JK, Wojtys EM. Long-term results after combined medial meniscal allograft transplantation and anterior cruciate ligament reconstruction: Minimum 8.5-year follow-up study. Arthroscopy 2004;20:129-140. 6. Wirth CJ, Peters G, Milachowski KA, Weismeier KG, Kohn D. Long-term results of meniscal allograft transplantation. Am J Sports Med 2002;30:174-181. 7. Hommen J, Applegate G, Del Pizzo W. Meniscus allograft transplantation: Ten-year results of cryopreserved allografts. Arthroscopy 2007;23:388-393. 8. Rue JP, Yanke AB, Busam ML, McNickle AG, Cole BJ. Prospective evaluation of concurrent meniscus transplantation and articular cartilage repair: Minimum 2-year follow-up. Am J Sports Med 2008;36:1770-1778. 9. Fischer SP, Fox JM, Del Pizzo W, et al. Accuracy of diagnoses from magnetic resonance imaging of the knee. A multi-center analysis of one thousand and fourteen patients. J Bone Joint Surg Am 1991;73: 2-10. 10. McCauley T. MR imaging evaluation of the postoperative knee. Radiology 2005;234:53-61. 11. Potter HG, Rodeo SA, Wickiewicz TL, Warren RF. MR imaging of meniscal allografts: Correlation with clinical and arthroscopic outcomes. Radiology 1996;198:509-514. 12. Verstraete KL, Verdonk R, Lootens T, et al. Current status and imaging of allograft meniscal transplantation. Eur J Radiol 1997;26: 16-22. 13. Lee DH, Kim TH, Lee SH, et al. Evaluation of meniscus allograft transplantation with serial magnetic resonance imaging during the first postoperative year: Focus on graft extrusion. Arthroscopy 2008;24:1115-1121. 14. Shelton WR, Dukes AD. Meniscus replacement with bone anchors: A surgical technique. Arthroscopy 1994;10:324-327. 15. Wilcox TR, Goble EM. Indications for meniscal allograft reconstruction. Am J Knee Surg 1996;9:35-36. 16. Stoller DW, Cannon WD Jr, Anderson LJ. The knee. In: Stoller DW, ed. Magnetic resonance imaging in orthopaedics and sports medicine. Ed 2. Philadelphia: Lippincott-Raven, 1997;203-442. 17. Rankin M, Noyes F, Barber-Westin S, Hushek S, Seow A. Human meniscus allografts’ in vivo size and motion characteristics: Magnetic resonance imaging assessment under weightbearing conditions. Am J Sports Med 2006;34:98-107. 18. Hantes M, Zachos V, Zibis A, et al. Evaluation of meniscal repair with serial magnetic resonance imaging: A comparative study between conventional MRI and indirect MR arthrography. Eur J Radiol 2004;50:231-237. 19. Arnoczky SP, Cooper TG, Stadelmaier DM, Hannafin JA. Magnetic resonance signals in healing menisci: An experimental study in dogs. Arthroscopy 1994;10:552-557. 20. Arnoczky SP, DiCarlo EF, O’Brien SJ, Warren RF. Cellular repopulation of deep-frozen meniscal autografts: An experimental study in the dog. Arthroscopy 1992;8:428-436. 21. Verdonk PC, Verstraete KL, Almqvist KF, et al. Meniscal allograft transplantation: Long-term clinical results with radiological and magnetic resonance imaging correlations. Knee Surg Sports Traumatol Arthrosc 2006;14:694-706.