Post-Arthroscopy Osteonecrosis of the Knee: With Special Reference to Meniscal Extrusion before and after the Medial Meniscectomy

Background: Post-arthroscopic osteonecrosis of the knee (PAONK) is a rare disorder condition. In addition, no studies have analyzed the relationship between the meniscus extrusion and PAONK. The purposes of this retrospective study are to report 10 cases of 7 PAONK and to evaluate location of the MM in the joint space, including the meniscus 8 extrusion, before and after the meniscectomy in these 10 knees. Methods: Ten knees with PAONK were found in 876 knees which had undergone 10 arthroscopic partial meniscectomy of the medial meniscus (MM). We retrospectively 11 collected the clinical, surgical, and radiological data of these 10 patients. To evaluate 12 intraarticular location of the menisci in the joint space, Extrusion width and Inner width 13 were defined on magnetic resonance images, and they were statistically compared. 14 Results: In the 10 patients (2 men and 8 women; the age ranged between 60 and 80 years), 15 knee pain recurred within 12 weeks after the meniscectomy. Preoperatively, the Extrusion 16 width of the MM (the mean, 4.7 mm) was significantly greater (P<0.0001) than that of 17 the LM (0.2 mm). At the time of diagnosis of PAONK, the Extrusion width of the MM significantly increased by 1.2 mm and the Inner width was significantly reduced by mm, compared with the preoperative values (P<0.0001). Conclusion: This is the first report that, in each of the 10 knees with PAONK, the MM 21 had been extruded before the meniscectomy, and then, the Inner width of the MM almost 22 disappeared postoperatively due to not only the increased meniscal extrusion but the 23 partial meniscal resection. This study suggested that the intraarticular abnormal location of the MM, including the meniscal extrusion, is a critical factor that should be evaluated 25 in future studies to clarify causative factors of PAONK.

arthroscopic surgery. Particularly, no studies have analyzed the relationship between the 50 meniscus extrusion and PAONK. Recently,Oda et al. [21] reported that there was a 51 significant correlation between the meniscal extrusion and the SPONK. Therefore, it is 52 required to analyze a relationship between the meniscal extrusion and the PAONK. 53 In our clinical practice, we found 10 knees with PAONK in 876 knees which had 54 undergone arthroscopic partial meniscectomy of the medial meniscus (MM). The first 55 purpose of the present study is to report the clinical status of the 10 patients in detail to 56 increase the database of PAONK. The second purpose is to clarify whether these 10 knees 57 had abnormal location of the MM in the joint space, including the meniscus extrusion, 58 before and after the arthroscopic partial meniscectomy.    86 The radiological stage of osteoarthritis (OA) was evaluated using the Kellgren and 87 Lawrence (KL) grades [13]. The radiological stage of osteonecrosis of the knee was 88 evaluated using Koshino's classification [14] (Table 2). In the present study, the MRI was

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The location of the MM in the joint space, including the degree of extrusion of 96 the MM, was quantified using the following method, which was created by modification 97 of Costa's method [7]. On the coronal image of the MRI at the midpoint of the medial 98 femoral condyle, a vertical line intersecting the peripheral margin of the medial tibial 99 plateau was drawn ( Figure 1). Osteophytes were excluded in determining the medial 100 margin. Then, the second and third lines, which were parallel to the first line, were drawn 101 at the outer (peri-articular) and inner (intra-articular) margins of the meniscus, 102 respectively ( Figure 1). We measured the distance between the first and second lines, 103 which was defined as "Extrusion width", and the distance between the first and third lines, 104 which was defined as "Inner width" (Figure 1). Each width was quantified in millimeters.

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This method was applied to the lateral joint space in the same manner to quantify the 106 location of the lateral meniscus (LM). Locations of the MM and LM in the joint space 107 was measured before the arthroscopic surgery and at the time when the diagnosis of 108 PAONK was made.   Table 3. After arthroscopic 139 meniscectomy, 9 of the 10 patients returned to their usual daily activities, although 3 of 140 the 9 patients felt some mild pain in the ipsilateral knee while walking. In these 9 patients, 141 serious knee pain recurred 3 to 12 weeks (the mean, 5.7 weeks) after the meniscectomy 142 so that each patient visited the outpatient clinic of our hospital. The remaining one patient 143 continuously felt moderate knee pain after the meniscectomy. The 10 patients underwent 144 an MRI examination 3 to 21 weeks (the mean, 12.0 weeks) after the meniscectomy. The

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MRI showed that an osteonecrosis lesion with BME was found at the MFC in 8 knees  Before the arthroscopic meniscectomy, the MRI showed that the intraarticular location of 155 the MM was abnormal in each of the 10 patients, while that of the LM was normal (Table   156 4). In the measurements, the Extrusion width of the MM averaged 4.7 mm with a range between 2.8 and 6.8 mm, which was significantly greater (P<0.0001) than that of the LM 158 (the mean, -0.2 mm). The Inner width of the MM averaged 4.4 mm with a wide range 159 between 0.0 and 7.1mm, which was significantly less (P<0.0001) than that of the LM (the First, there was no difference in the incidence between men and women. Secondly, the 212 average age at the onset in their 47 cases was 58 years, which was younger compared 213 with that for SPONK. Thirdly, PAONK occurred after medial meniscectomy in 87% and 214 after lateral meniscectomy in 13%. Fourthly, PAONK occurred at the MFC in 83% of the 215 cases, at the lateral femoral condyle in 8.5%, at the MTP in 4.3%, and at the lateral tibial 216 plateau in 4.3%. Fifthly, articular cartilage injuries were found in 65% of the patients. In 217 the present study, the clinical characteristics of the 10 patients with PAONK almost agreed 218 with those reported by Pope except for the following points: (1) The age of our patients 219 (67.5 years in average) was higher by approximately 10 years in comparison with Pape's 220 report.
(2) Cartilage injuries were observed before the meniscectomy in all patients. These 221 differences may reflect a fact that the aging rate in Japan, currently 27.3%, is the highest 222 in the world. In the present study, 90% of the patients with PAONK felt severe pain within 223 6 months after MM resection. Therefore, it is recommended that, when a patient who but a necrotic bone tissue, which appeared to be a secondary lesion to the fracture, was 274 found in only two knees. Then, in the 10 knees of the present study, the MRI taken in the 275 early phase after recurrence of the knee pain showed that the BME lesion, which is 276 commonly seen around an insufficiency fracture lesion, was widely observed in the MFC 277 and/or the MTP. In the late phase after recurrence of the knee pain, a low intensity zone 278 in both the T1-and T2*-weighted images, which showed osteonecrosis, was observed at 279 only a localized area in the subchondral bone. These findings in the present study 280 supported the above-described histopathological studies.

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Then, we discuss about pathological difference between PAONK and SPONK.

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In 2000, Yamamoto et al. [26] reported that the primary event leading to SPONK is a 283 subchondral insufficiency fracture, and that the localized necrosis of the bone tissue seen 284 in association with SPONK is a secondary lesion following the fracture of the subchondral 285 bone tissue. It is noted that this pathology of SPONK is the same as the above-described

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This is a report dealing with 10 rare cases of PAONK. Therefore, there are 297 limitations in the present study. First, the number of patients was insufficient. Secondly, 298 there were no control data to clarify the effect of the abnormal location of the MM on 299 occurrence of PAONK. However, we believe that this report will contribute to future 300 studies, because this is the first report that abnormal location of the MM, including 301 meniscal extrusion, was observed in each of the 10 patients not only at the time of 302 diagnosis of PAONK but before the prior meniscectomy.    The datasets generated and/or analysed during the current study are available from 332 the corresponding author on reasonable request.        were drawn at the outer and inner margins of the meniscus, respectively. We measured 433 the distance between the first and second lines, which was defined as "Extrusion width 434 (EW)", and the distance between the first and third lines, which was defined as "Inner 435 width (IW)". Each width was quantified in millimeters.      show an increase and a decrease of the width, respectively. a: comparison between the MM and the LM (unpaired t-test). b: comparison between the 2 538 periods (paired t-test).     Measurement of the location of the medial meniscus (MM) on the coronal image of the MRI. A vertical line intersecting the peripheral margin of the medial tibial plateau (MTP) was drawn. Then, the second and third lines, which were parallel to the rst line, were drawn at the outer and inner margins of the meniscus, respectively. We measured the distance between the rst and second lines, which was de ned as "Extrusion width (EW)", and the distance between the rst and third lines, which was de ned as "Inner width (IW)". Each width was quanti ed in millimeters. Figure 2 62-year old woman. Preoperatively, the radiogram (A) showed grade-1 OA. The MR images (B and C: T2*, D: T1) did not show any ndings of osteonecrosis or bone marrow edema (BME). The Inner width (IW) and the Extrusion width (EW) of the MM were 6.1 mm and 3.5 mm, respectively (B). At 3 months postoperatively, the radiograms (E) indicated stage-3 osteonecrosis in the medial femoral condyle (MFC).
MR images (F and G: T2*, H: T1) showed a necrotic lesion (T1-low, T2*-high) in the MFC, which was surrounded by an osteosclerotic zone and a wide BME area. The IW decreased to 1.4 mm, and the EW increased to 6.1 mm (F). 64-year old man. Preoperatively, the radiogram (A) showed grade-2 OA. The MR images (B and C: T2*, D: T1) did not show any ndings of osteonecrosis or bone marrow edema (BME). The Inner width (IW) and the Extrusion width (EW) of the MM were 6.9 mm and 4.1 mm, respectively (B). At 4 months postoperatively, the radiograms (E) indicated stage-2 osteonecrosis in the medial femoral condyle (MFC). MR images (F and G: T2*, H: T1) showed a necrotic lesion (T1-low, T2*-high) in the MFC, which was surrounded by an osteosclerotic zone. The IW decreased to 1.2 mm, while the EW was 4.2 mm (F).

Figure 4
A: Comparison of the medial meniscus (MM) locations measured before the meniscectomy (Preop) and at the time of diagnosis of PAONK (@Diagnosis). The Extrusion width (EW) and the Inner width (IW), the de nition of which is shown in a schematic picture (B), were signi cantly (P<0.0001) changed between the 2 periods. This bar graph also shows that the disappearance of the IW of the MM was caused by not only the decrease of the total meniscal width due to meniscectomy but also the meniscal extrusion.