We thank Dr. Pape and colleagues for their questions regarding the interesting and challenging dilemma when we face patients who present with pain after knee arthroscopic surgery. In response to their questions, no patient in our cohort reported a traumatic episode to the knee that could have accounted for the presence of a subchondral bone fracture.

All patients in this series had normal bone marrow signal on pre-operative MR imaging which had been obtained after more than 6 weeks of symptoms. Following arthroscopic meniscectomy,there was increased bone marrow edema (BME) noted following onset of pain. All of the patients were more than six weeks post arthroscopy when they underwent post -arthroscopy MRI. Although the concern raised by Dr.Pape et al. is valid, the window period does not seem relevant in this study as this issue is only important if symptoms were present with normal bone marrow signal on MR imaging when assessing the pre-operative meniscal tear and excluding bone marrow pathology.

Furthermore, the pathology provides a tissue diagnosis which is more definitive than MRI findings. This is an important point with regard to the comment that “this contradicts some studies that have proven the existence of osteonecrosis (3,6).” All of these previous studies have made the presumption that BME on MR imaging EQUALS osteonecrosis. A diagnosis of osteonecrosis can only be confirmed on pathologic examination by the presence of cell death.

Nevertheless, our study does not exclude the existence of ONPK or SPONK but suggests that fracturing of the subchondral bone plate is the primary pathology. Thus, assigning “bone death” from vascular insufficiency as a cause in every patient with these MRI findings is not accurate.

We encourage further histopathologic analyses of patients who have this condition after knee arthroscopy and and require subsequent knee replacement surgery to support or refute our findings. We agree that similar treatment for both early and late stages of this condition is the same, whether subchondral fracture or osteonecrosis is diagnosed. Prospective, randomized studies assessing the role of adjuvant medical therapies or protected weight bearing after surgery are required to prevent its occurrence, and for this to occur, identification of high-risk patients is critical. Who is high risk and how we treat them is not yet defined.

To the Editor:

In their recent article (1), MacDessi et al. concluded that postarthroscopic changes following knee arthroscopy were attributable to a subchondral fracture rather than to osteonecrosis. To justify this conclusion, I would suggest that the answers to the following questions are critical.

1. Have they ruled out intervening trauma to the knee between the time of arthroscopy and total knee arthroplasty?

2. Pre-existing, undiagnosed early-stage spontaneous osteonecrosis of the knee (SPONK) is the most important differential diagnosis when considering the etiology of osteonecrosis in the postoperative knee (ONPK) (2). An interval of 6 weeks between the onset of osteonecrosis-induced symptoms and its appearance on MRI has been noted (“window period”) of the MRI method to detect SONK)(3-5). Had the diagnostic window of the MRI method to detect SPONK been respected in this study ?

The conclusion of this study essentially questions the existence of both osteonecrosis in the postoperative knee (ONPK) and spontaneous osteonecrosis of the knee (SPONK). This contradicts some studies that have proven the existence of osteonecrosis (3,6). However, it is well known that contradictory reports exist about the histologic findings of the resected bone and cartilage in ONPK patients.

To get an unbiased view on the question of etiology in ONPK it is useful to refer to the following questions as KIDWAI did for SPONK (7):

1. Does ONPK exist? The answer is "probably," but much more rarely than initially believed (2).

2. Will different etiologies influence the treatment? The answer appears to be "no" because subchondral fractures and SPONK are treated alike: conservatively in early stages and surgically if changes of subchondral surface collapse are present.

3. Is there any method of halting the progression of subchondral changes ? The answer is "maybe" since non-weight-bearing therapy has been associated with lack of progression of the disease in some cases.

4. Is it important to rule out preexisting SPONK in patients with postarthroscopic MRI changes ? The answer is yes for two reasons: a) Medicolegally, arthroscopy could be wrongly regarded as the primary cause of osteonecrosis. b) A distinction between “true” PAON and SPONK is important to avoid unnecessary repeat arthroscopy. To avoid medicolegal implications, the uncontroversial term “osteonecrosis or subchondral fracture in the postoperative knee” should be used.

The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

References

1. MacDessi S, Brophy R, Bullough P, Windsor R, Sculco T. Subchondral fracture following arthroscopic knee surgery. J Bone Joint Surg Am. 2008;90:1007-12.

2. Pape D, Seil R, Anagnostakos K, Kohn D. Postarthroscopic osteonecrosis of the knee. Arthroscopy. 2007;23:428-38.

3. Johnson TC, Evans JA, Gilley JA, DeLee JC. Osteonecrosis of the knee after arthroscopic surgery for meniscal tears and chondral lesions. Arthroscopy. 2000;16:254-61.

4. Muscolo DL, Costa-Paz M, Ayerza M, Makino A. Medial meniscal tears and spontaneous osteonecrosis of the knee. Arthroscopy. 2006;22:457-60.

5. Marmor L, Goldberg RT. Failure of magnetic resonance imaging in evaluating osteonecrosis of the knee. Am J Knee Surg. 1992;5:195-201.

6. al Kaar M, Garcia J, Fritschy D, Bonvin JC. [Aseptic osteonecrosis of the femoral condyle after meniscectomy by the arthroscopic approach]. J Radiol. 1997;78:283-88. French.

7. Kidwai AS, Hemphill SD, Griffiths HJ. Radiologic case study. Spontaneous osteonecrosis of the knee reclassified as insufficiency fracture. Orthopedics. 2005;28:236, 333-36.