JBJS | Spontaneous Osteonecrosis of the Knee: The Result of Subchondral Insufficiency Fracture*

The Journal of Bone & Joint Surgery, Volume 82, Issue 6

Articles | June 01, 2000

Spontaneous Osteonecrosis of the Knee: The Result of Subchondral Insufficiency Fracture*

Takuaki Yamamoto, M.D., Ph.D.†; Peter G. Bullough, M.B., Ch.B.‡

Investigation performed at the Department of Orthopaedic Pathology, The Hospital for Special Surgery, New York, N.Y.
*Although none of the authors has received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article, benefits have been or will be received but are directed solely to a research fund, foundation, educational institution, or other nonprofit organization with which one or more of the authors is associated. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were a Research Fellowship from Sumitomo Life Social Welfare Services Foundation and a Grant-in-Aid for JSPS Fellows.
†Department of Orthopaedic Surgery, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582 Japan. E-mail address: yamataku@ortho.med.kyushu-u.ac.jp.
‡Department of Laboratory Medicine, The Hospital for Special Surgery, 535 East 70th Street, New York, N.Y. 10021. E-mail address: bulloughp@hss.edu.

The Journal of Bone & Joint Surgery. 2000; 82:858-858

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Abstract

Background: Spontaneous osteonecrosis of the knee is a superficial subchondral lesion classically seen in the medial femoral condyle; in general, it is markedly different in its clinicopathological presentation from the classic wedge-shaped subchondral osteonecrotic lesions seen in the hip, knee, and other joints. Recent reports on subchondral insufficiency fracture of the femoral head, which has marked morphological similarities with spontaneous osteonecrosis of the knee, led us to reevaluate a series of patients who had had operative treatment because of a clinical and pathological diagnosis of spontaneous osteonecrosis of the knee.

Methods: We reviewed the cases of fourteen patients who had had operative treatment of spontaneous osteonecrosis of the knee in order to reevaluate the gross and histological morphology of this lesion. The patients included eight women and six men who ranged in age from fifty-nine to eighty-eight years. In all patients, the diagnosis of spontaneous osteonecrosis of the knee had been based on clinical presentation, imaging studies, and pathological findings. The appearance of the lesion on plain radiographs was categorized into four stages, which corresponded to the gross and histological findings. In stage 1, the radiographic appearance is normal; in stage 2, a radiolucent oval area is seen subchondrally or there is slight flattening of the convexity of the condyle, or both; in stage 3, the radiolucent area is expanded and is surrounded by a sclerotic halo; and in stage 4, secondary osteoarthritic changes are apparent.

Results: No patient had a stage-1 lesion. Three patients, all of whom had a stage-2 lesion, were considered to have a subchondral insufficiency fracture of the medial femoral condyle. Another six patients, all of whom had a stage-3 lesion, were considered to have a subchondral fracture and associated focal osteonecrosis that was confined to the area between the fracture line and the articular surface. The remaining five patients, three of whom had a stage-3 lesion and two of whom had a stage-4 lesion, had indeterminate findings because the lesion had become detached from the condyle.

Conclusions: Our histopathological findings suggest that the primary event leading to spontaneous osteonecrosis of the knee is a subchondral insufficiency fracture and that the localized osteonecrosis seen in association with this disease is the result of a fracture.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | References

Spontaneous osteonecrosis of the knee has been recognized as a distinct form of osteonecrosis since it was first described in 1968³. The lesion is clinically characterized by the sudden onset of severe knee-joint pain in older patients and is not usually associated with systemic disorders or previous corticosteroid therapy. In general, the lesion is immediately subarticular and is located in the medial femoral condyle. In the early period after the onset of pain, the radiographic findings are usually unremarkable^{1,3,4,10-12,16,18,19}.

Ahlbück et al.³, in the first clinical report of which we are aware, reported on forty patients (six men and thirty-four women) who were an average of seventy years old. In that study, the diagnosis of spontaneous osteonecrosis of the knee was mainly based on a focal increase in isotope uptake confined to one side of the joint and on the exclusion of other known clinical entities. Seven patients had a needle biopsy, and the histopathological findings confirmed the diagnosis of osteonecrosis³. In 1978, Ahuja and one of us (P. G. B.) reported on the histopathological appearance of osteonecrosis of the knee in a study of thirty-four specimens that had been removed at the time of total knee replacement⁴. In the twelve knees that were considered to have spontaneous osteonecrosis, the lesion involved only a small area of subchondral tissue, whereas in the twenty-two knees that had classic osteonecrosis in association with corticosteroid therapy or rheumatoid arthritis, the lesion was larger and wedge-shaped. However, at that time, the etiology of spontaneous osteonecrosis of the knee was not questioned.

Recently, subchondral insufficiency fractures of the femoral head have been described in retrospective studies of lesions that were previously diagnosed as osteonecrosis^23,24,26. Insufficiency fractures occur in bones that have been mechanically weakened by nontumorous disease. It has been suggested that the proximate cause of such fractures is bone insufficiency resulting from osteoporosis^{5,7,17,21,23,24,26}. In the retrospective studies just mentioned^23,24,26, the foci of osteonecrosis were considered to be secondary lesions resulting from fracture. For this reason, we reevaluated a series of patients who had been treated operatively because of a clinical and pathological diagnosis of spontaneous osteonecrosis of the knee.

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Fig. 1:: Diagrams depicting the histopathological classification of the lesion. Patients in Group I had a subchondral fracture with no evidence of antecedent osteonecrosis. Patients in Group II had a subchondral fracture with associated focal osteonecrosis that was confined to the region between the fracture line and the articular surface. Patients in Group III had indeterminate findings because the articular cartilage and underlying tissue had become detached from the femoral condyle; in such cases, there is usually no residual necrotic tissue to be found in the bone.

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Fig. 2-A:: Figs. 2-A through 2-D: Case 3, a seventy-two-year-old man who had a sudden onset of knee pain without any history of antecedent trauma.
Fig. 2-A: Anteroposterior standing radiograph of the right knee, made seven months after the onset of pain, showing subchondral collapse in the medial femoral condyle and focal areas of radiolucency. The lesion was classified as stage 2.

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Fig. 2-B::

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Fig. 2-C:: Magnetic resonance images made seven months after the onset of pain. The T1-weighted image (Fig. 2-B) demonstrates diffuse low signal intensity in the medial femoral condyle, with an associated focal band of lower signal intensity in the region of the lesion (arrow). The gradient-echo image (Fig. 2-C) demonstrates diffuse high signal intensity in the corresponding area, with a focal area of low signal intensity underlying the articular cartilage.

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Fig. 2-D:: Photomicrograph of a specimen obtained from the area beneath the articular cartilage, demonstrating fractured bone trabeculae with associated fracture callus and vascular granulation tissue, without any evidence of antecedent osteonecrosis (hematoxylin and eosin, 220).

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Fig. 3-A:: Figs. 3-A through 3-D: Case 8, a seventy-two-year-old woman who had a sudden onset of pain without any history of antecedent trauma.
Fig. 3-A: Anteroposterior standing radiograph of the left knee, made eleven months after the onset of pain, demonstrating subchondral collapse in the medial femoral condyle. Both radiolucent and sclerotic areas are evident, and a sclerotic halo is apparent. The lesion was classified as stage 3.

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Fig. 3-B:: Magnetic resonance images made eleven months after the onset of pain. The T1-weighted image (Fig. 3-B) demonstrates diffuse low signal intensity in the medial femoral condyle, with an associated band of lower signal intensity in the area of the lesion. The T2-weighted image (Fig. 3-C) demonstrates an inhomogeneous area of high signal intensity in the corresponding region and a focal area of low signal intensity underlying the articular cartilage.

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Fig. 3-C::

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Fig. 3-D:: Photomicrograph demonstrating a fracture line in the subchondral area, paralleling the subchondral osseous end plate. A focal area of necrosis (N) is apparent between the fracture line and the articular surface. Repair tissue consisting of fracture callus (arrows) and granulation tissue (asterisk) is apparent around the necrotic area. The surrounding bone tissue is being focally resorbed by active osteoclasts and replaced by vascular granulation tissue (asterisk), which corresponds to the lytic area on plain radiographs. These repair tissues are characteristically found at the site of fracture repair, suggesting that fracture is the primary event (hematoxylin and eosin, 180).

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TABLE I: Clinicopathological Characteristics of the Fourteen Patients

*In women, the ideal body-mass index is approximately 22.5 and obesity is defined as a body-mass index of 27.0 or more. In men, the ideal body-mass index is approximately 23.3 and obesity is defined as a body-mass index of 28.0 or more².†Assistive devices included a crutch, walker, or wheelchair.‡The patient received an injection of a corticosteroid into the knee joint after the onset of knee pain.

Case	Gender, Age (yrs.)	Side	Height (m)	Weight (kg)	Body-Mass Index*(kg/m²)	Steroid Intake	Alcohol Abuse	Radiographic Stage	Degree of Pain	Treatment†	Time Between Onset of Pain and Op. (mos.)	Associated Conditions
Group I (subchondral fracture)
1	F, 83	L	1.45	73	34.7	No	No	2	Severe	Anti-inflammatory drugs, assistive device	3
2	M, 61	R	1.77	82	26.2	No	No	2	Severe	Anti-inflammatory drugs, assistive device	6	Bilateral femoral neck fracture 3 mos. after op.
3	M, 72	R	1.73	109	36.4	No	No	2	Moderate	Anti-inflammatory drugs, assistive device	7
Group II (subchondral fracture with osteonecrosis)
4	F, 69	R	1.60	77	30.1	No	No	3	Severe	Anti-inflammatory drugs	4
5	F, 70	R	1.57	68	27.6	No	No	3	Severe	Anti-inflammatory drugs, assistive device	4	Right femoral neck fracture 1 yr. after op.
6	F, 73	R	1.62	73	27.8	No	No	3	Severe	Anti-inflammatory drugs, assistive device	5
7	F, 75	L	1.50	74	32.9	No	No	3	Moderate	Anti-inflammatory drugs, assistive device	5
8	F, 72	L	1.53	60	25.6	No	No	3	Moderate	Anti-inflammatory drugs	12
9	M, 71	L	1.68	75	26.6	No	No	3	Moderate	Anti-inflammatory drugs	8
Group III (indeterminate)
10	F, 76	L	1.52	84	36.4	No	No	3	Severe	Anti-inflammatory drugs, assistive device	8
11	F, 88	R	1.60	56	21.9	No	No	3	Moderate	Anti-inflammatory drugs, assistive device	5
12	M, 86	R	1.68	68	24.1	No	No	3	Moderate	Anti-inflammatory drugs, assistive device	24
13	M, 70	R	1.78	79	24.9	No‡	No	4	Moderate	Anti-inflammatory drugs, corticosteroid injection after onset of pain	10
14	M, 59	L	1.75	81	26.4	No	No	4	Moderate	Anti-inflammatory drugs	13

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TABLE I: Clinicopathological Characteristics of the Fourteen Patients

Case	Gender, Age (yrs.)	Side	Height (m)	Weight (kg)	Body-Mass Index*(kg/m²)	Steroid Intake	Alcohol Abuse	Radiographic Stage	Degree of Pain	Treatment†	Time Between Onset of Pain and Op. (mos.)	Associated Conditions
Group I (subchondral fracture)
1	F, 83	L	1.45	73	34.7	No	No	2	Severe	Anti-inflammatory drugs, assistive device	3
2	M, 61	R	1.77	82	26.2	No	No	2	Severe	Anti-inflammatory drugs, assistive device	6	Bilateral femoral neck fracture 3 mos. after op.
3	M, 72	R	1.73	109	36.4	No	No	2	Moderate	Anti-inflammatory drugs, assistive device	7
Group II (subchondral fracture with osteonecrosis)
4	F, 69	R	1.60	77	30.1	No	No	3	Severe	Anti-inflammatory drugs	4
5	F, 70	R	1.57	68	27.6	No	No	3	Severe	Anti-inflammatory drugs, assistive device	4	Right femoral neck fracture 1 yr. after op.
6	F, 73	R	1.62	73	27.8	No	No	3	Severe	Anti-inflammatory drugs, assistive device	5
7	F, 75	L	1.50	74	32.9	No	No	3	Moderate	Anti-inflammatory drugs, assistive device	5
8	F, 72	L	1.53	60	25.6	No	No	3	Moderate	Anti-inflammatory drugs	12
9	M, 71	L	1.68	75	26.6	No	No	3	Moderate	Anti-inflammatory drugs	8
Group III (indeterminate)
10	F, 76	L	1.52	84	36.4	No	No	3	Severe	Anti-inflammatory drugs, assistive device	8
11	F, 88	R	1.60	56	21.9	No	No	3	Moderate	Anti-inflammatory drugs, assistive device	5
12	M, 86	R	1.68	68	24.1	No	No	3	Moderate	Anti-inflammatory drugs, assistive device	24
13	M, 70	R	1.78	79	24.9	No‡	No	4	Moderate	Anti-inflammatory drugs, corticosteroid injection after onset of pain	10
14	M, 59	L	1.75	81	26.4	No	No	4	Moderate	Anti-inflammatory drugs	13

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Diagnostic Criteria

The present study is based on a histopathological reevaluation of fourteen patients who had had operative treatment of spontaneous osteonecrosis of the knee at our institution during the three-year period between January 1, 1995, and December 31, 1997. The diagnosis of spontaneous osteonecrosis of the knee had been based on clinical presentation, imaging studies (including plain radiography and magnetic resonance imaging in all cases and bone scintigraphy in two cases), and pathological findings^{3,4,10,12,16,18,19}.

The clinical diagnosis was based mainly on the criteria established in the report by Ahlbück et al.³ and the review article by Lotke and Ecker¹². These criteria include the sudden onset of severe pain in the knee joint in older subjects (defined as those who are more than sixty years old). In general, the patients have no associated systemic disorders and no history of corticosteroid therapy and the lesion is located in a superficial subchondral region in the medial femoral condyle.

In the present study, alcohol abuse was defined as consumption of more than 400 milliliters of ethanol per week, which has been reported to result in a high risk of osteonecrosis¹⁴. Ethanol consumption was determined according to the ethanol concentration in each type of beverage (presumed to be 4.5 milliliters per deciliter in beer and 40.0 milliliters per deciliter in hard liquor)¹⁴. The degree of pain (mild, moderate, or severe) was determined with use of a questionnaire, developed at our hospital, that was completed by the patient at the time of admission. The body-mass index, calculated as the body weight in kilograms divided by the height in meters squared, was used to determine whether the patient was obese. In the United States, the ideal body-mass index is considered to be approximately 23.3 for men and 22.5 for women and obesity is defined as a body-mass index of 28.0 or more for men and of 27.0 or more for women².

Spontaneous osteonecrosis of the knee has been categorized into four radiographic stages¹⁰. In stage 1, the radiographic appearance is normal; in stage 2, there is a radiolucent oval area in the subchondral region or slight flattening of the convexity of the condyle, or both; in stage 3, the radiolucent area is expanded and is surrounded by a sclerotic halo; and in stage 4, there are secondary osteoarthritic changes, including narrowing of the joint space, sclerosis, formation of osteophytes, and secondary sclerotic and destructive changes on the tibial side of the joint.

The magnetic resonance imaging criteria that were used for the diagnosis of spontaneous osteonecrosis of the knee were described in previous reports^12,18. The normal intensity on T1-weighted images is replaced by a discrete area of low signal intensity in the affected medial femoral condyle, with a focal area of lower signal intensity in the region of the lesion. T2-weighted and fat-suppression images typically show high signal intensity in the corresponding area, with an associated focal area of low signal intensity in the center of the lesion (a bone-marrow-edema pattern with an associated focal area of low signal intensity).

On bone scintigraphy, spontaneous osteonecrosis of the knee is defined as a focally intense area of uptake over the affected femoral condyle³.

The histopathological criteria that were used for the diagnosis of spontaneous osteonecrosis of the knee were described in a previous report⁴. Typically, there is a shallow subchondral fracture in the medial femoral condyle, with associated foci of trabecular and bone-marrow necrosis. The necrosis is localized to the area of the fracture and generally is surrounded by a band of fibrovascular granulation tissue of variable thickness, with focal areas of osteoclastic activity and histiocytic resorption of the dead tissue.

Tissue Preparation

All specimens were fixed in 10 percent formalin solution immediately after removal. Four-millimeter-thick coronal sections were obtained from the center of each lesion with use of a band saw. After decalcification in 5 percent nitric acid, the sections were processed and were embedded in paraffin. Seven-micrometer-thick full slab sections then were prepared; the sections were routinely stained with hematoxylin and eosin.

Histopathological Reevaluation

All fourteen patients were reevaluated histopathologically. The morphological criteria that were used for the diagnosis of a subchondral insufficiency fracture were based on previously described histopathological characteristics^23,24,26. The notable gross findings included the absence of the classic wedge-shaped infarct and the presence of a linear notched zone paralleling the subchondral osseous end plate, which is usually whitish-gray in color. Histologically, the whitish-gray area consists of irregularly arranged fracture callus, reactive cartilage, and granulation tissue^{6,20,23,24,26}. The histopathological evaluations were performed independently by both authors, whose primary interest is orthopaedic pathology.

Results

Introduction | Materials and Methods | Results | Discussion | References

Clinical Findings (Table I)

The study group included eight women and six men who ranged in age from fifty-nine to eighty-eight years (average, seventy-three years). The right knee was affected in eight patients, and the left knee was affected in six. There were no instances of bilateral disease. No patient had a history of corticosteroid intake or alcohol abuse. In general, symptoms were characterized by the sudden onset of pain without any antecedent trauma or preexisting disease. Six patients had severe pain at the time of admission, and eight had moderate pain. Thirteen patients were considered to be overweight on the basis of the body-mass index, and, of these, seven were considered to be obese. Two patients had a history of a fracture of the hip. One of these patients (Case 2) had a bilateral femoral neck fracture three months after the knee operation, and the other (Case 5) had a fracture of the right femoral neck one year after the procedure. No patient had evidence of osteonecrosis in any other joint.

All patients initially had been managed with nonoperative therapy, which included the use of nonsteroidal anti-inflammatory medications (all patients) and the use of a crutch, walker, or wheelchair (nine patients). One patient (Case 13) had been managed with the injection of a corticosteroid into the affected knee joint after the onset of pain. In all patients, progressive worsening of the pain had led to operative intervention. The length of time between the onset of knee pain and the operation ranged from three to twenty-four months.

Imaging and Histopathological Findings

The patients were classified into three histopathological groups on the basis of the morphological characteristics of the lesion (Fig. 1). Group I included patients who had a primary subchondral fracture, with no evidence of antecedent osteonecrosis. Group II included those who had a subchondral fracture with associated focal osteonecrosis that was confined to the region between the fracture line and the articular surface. Group III included those who had indeterminate findings because the articular cartilage and underlying tissue had become detached from the femoral condyle, leaving a shallow depression lined by fibrocartilaginous tissue. In such cases, it is not possible to demonstrate either the fracture or the necrosis because the detached flap of cartilage and subchondral bone usually are not available for study.

Group I (Subchondral Fracture)

Group I included three patients (two men and one woman) in whom the previous diagnosis of spontaneous osteonecrosis of the knee was revised to a diagnosis of subchondral insufficiency fracture of the medial femoral condyle.

Radiographically, all three lesions were classified as stage 2 (Fig. 2-A). In all cases, magnetic resonance imaging showed a pattern of bone-marrow edema with diffuse low signal intensity on T1-weighted images (Fig. 2-B) and high signal intensity on T2-weighted or fat-suppression or gradient-echo images (Fig. 2-C). A focal band of lower signal intensity was observed on the T1-weighted images. Bone scintiscans had not been made for any of the patients.

Histopathologically, a linear fracture line with associated fracture callus, reactive cartilage, and granulation tissue was observed at a depth of 0.1 to 0.3 centimeter from the articular surface. No histological evidence of antecedent osteonecrosis was observed (Fig. 2-D). The repair tissue was observed on both sides of the fracture line. The focal area of lower signal intensity on T1-weighted magnetic resonance images corresponded with the fracture line and the associated repair tissue. All patients had thin, disconnected bone trabeculae outside the area of the fracture, consistent with osteopenia.

Group II (Subchondral Fracture with Osteonecrosis)

Group II included six patients (one man and five women) who had a subchondral fracture with an associated area of osteonecrosis between the fracture line and the articular surface.

Radiographically, all six lesions were classified as stage 3 (Fig. 3-A). In all cases, magnetic resonance imaging showed a pattern of bone-marrow edema with diffuse low signal intensity on T1-weighted images (Fig. 3-B) and high signal intensity on T2-weighted or fat-suppression images (Fig. 3-C). A focal area of lower signal intensity was observed on the T1-weighted images. Bone scintigraphy, which had been performed for one patient, showed increased uptake in the medial femoral condyle.

Histopathologically, a linear fracture line with associated chondral metaplasia, fracture callus, and granulation tissue was observed at a depth of 0.2 to 0.5 centimeter from the articular surface. A focal area of necrosis was observed distal to the fracture line (that is, between the fracture line and the articular surface) (Fig. 3-D). However, the necrosis was minimal and was localized to the area of the fracture (unlike the morphological appearance of classic osteonecrosis of the knee), suggesting that the fracture was the primary event. The area of lower signal intensity on T1-weighted images corresponded with the fracture line and the associated repair tissue that was seen histologically. The surrounding bone tissue was being focally resorbed by active osteoclasts and replaced by vascular granulation tissue, which corresponded to the lytic area on plain radiographs. All patients had thin, disconnected bone trabeculae in the epiphyseal region.

Group III (Indeterminate Findings)

Group III included five patients (three men and two women) in whom it was not possible to detect either a fracture or necrosis.

Radiographically, three lesions were classified as stage 3 and two were classified as stage 4. Magnetic resonance imaging showed a pattern similar to that observed in Groups I and II. Bone scintigraphy, which had been performed for one patient with a stage-3 lesion, showed increased uptake in the medial femoral condyle.

Histopathological studies revealed that the affected area had been replaced by fibrocartilaginous tissue in two patients. The other three patients had saucerized defects on the surface of the femoral condyle, where the articular cartilage and subchondral bone were absent (presumably as a result of the formation of a flap and its subsequent release into the joint space). The bed of the defect was covered to various degrees by fibrocartilaginous tissue. In our opinion, these findings represent a more advanced histopathological stage than that observed in Group II.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

Our recent awareness of a previously unrecognized morbidity of the hip, subchondral insufficiency fracture of the femoral head^{5,7,17,21,23,24,26}, led us to reexamine the cases of fourteen patients who had had operative treatment of spontaneous osteonecrosis of the knee. In what we believe to have been the first clinical report on this topic³, the diagnosis of spontaneous osteonecrosis of the knee was mainly based on a focally increased uptake of isotope confined to one side of the joint and on the exclusion of other clinical entities, including osteochondritis dissecans, osteoarthritis, fracture, infection, neuropathy, and tumor. The diagnosis of osteonecrosis was supported by the histopathological findings for seven patients who had a needle biopsy³.

The cause of a disease is generally more obvious in the early stages of the disease process, when secondary changes are minimal. In the three patients in the present study who had a stage-2 lesion (the earliest type of lesion that was available for review), the most obvious histological finding was a subchondral fracture without any evidence of antecedent osteonecrosis. In these three patients, the diagnosis of spontaneous osteonecrosis of the knee had been based on previous reports related to this condition^3,4,10,12. Subchondral insufficiency fracture of the medial femoral condyle was only recently recognized as a morbid condition and therefore was not considered at the time of diagnosis.

All classic wedge-shaped osteonecrotic lesions that have been treated in our department have been complicated by fracture. In general, these fractures are not associated with fracture callus because they involve dead bone. In the three patients in the current study who had a stage-2 lesion, the diagnosis of spontaneous osteonecrosis of the knee had been made because of the presence of a fracture and tiny foci of fracture-induced bone necrosis, despite the lack of an obvious osteonecrotic lesion in the examined sections. It is important to keep in mind that necrosis of the immediately adjacent bone and marrow tissue is inevitable following any fracture^6,20,24,26. What we had failed to notice previously was the presence of fracture callus around the fracture line.

The lack of immobilization and continued weight-bearing following a subchondral fracture could eventually result in further displacement, subchondral collapse, and subsequent extension of necrosis^22-26. In Group II, the necrotic areas were present only between the fracture line and the articular surface and therefore appeared to be the result of the fracture. In Group III, the shallow defect of the medial femoral condyle was the result of detachment of the fracture fragment. On the basis of our reevaluation of the histological specimens, we are convinced that the osteonecrosis that was observed in our patients was the result of a subchondral fracture. However, we wish to make it clear that we do not regard all cases of osteonecrosis of the knee to be the result of a subchondral insufficiency fracture.

In our experience, spontaneous osteonecrosis of the knee is less common and has a very different morphology than classic osteonecrosis of the knee⁴. The classic nontraumatic form of osteonecrosis of the knee has been associated with various factors, especially corticosteroid intake. It is often bilateral, frequently involves large portions of the epiphysis and metaphysis, and usually is apparent on plain radiographs at the time of the onset of symptoms^4,8,9,13,15. In contrast, spontaneous osteonecrosis of the knee is generally characterized by a shallow lesion involving the medial femoral condyle. It typically occurs in older patients (average age, seventy years), most of whom have only a single focus. The initial findings on plain radiographs are inconspicuous despite the presence of severe pain; therefore, magnetic resonance imaging or bone scintigraphy, or both, currently are necessary for diagnosis^{1,3,4,10-12,16,18,19}.

Lotke et al.¹¹ proposed that microfractures within osteoporotic subchondral bone may be a possible etiological mechanism leading to osteonecrosis of the knee. In the present study, histopathological analysis revealed thin, disconnected bone trabeculae in the epiphysis of the medial femoral condyle. We hypothesize that the subchondral insufficiency fractures resulted from underlying osteoporosis. This view is consistent with the history of an acute onset of pain and the fact that many of the reported cases of spontaneous osteonecrosis of the knee occurred in older patients who had relatively osteoporotic bone^3,4,11,12.

Since our primary goal was to present histological confirmation of this condition, the present study only included patients who had been managed operatively. Several authors who have investigated the natural history and outcome of this disease have reported that the size and width of the lesion are important and that the prognosis is benign if the lesion is small^{1,11,12,16,19}.

In conclusion, our histopathological findings suggest that subchondral insufficiency fracture resulting from underlying osteoporosis is the etiology of spontaneous osteonecrosis of the knee.

References

Introduction | Materials and Methods | Results | Discussion | References

Aglietti, P.; Insall, J. N.; Buzzi, R.; and Deschamps, C.: Idiopathic osteonecrosis of the knee. Aetiology, prognosis and treatment. J. Bone and Joint Surg.,65-B(5): 588-597, 1983.65-B(5)588 1983

Agras, W. S.: Obesity. In Scientific American Medicine, pp. 1-9. Edited by D. C. Dale and D. D. Federman. New York, Scientific American, 1998.

Ahlbück, S.; Bauer, G. C.; and Bohne, W. H.: Spontaneous osteonecrosis of the knee. Arthrit. and Rheumat.,11: 705-733, 1968.11705 1968

Ahuja, S. C., and Bullough, P. G.: Osteonecrosis of the knee. A clinicopathological study in twenty-eight patients. J. Bone and Joint Surg.,60-A: 191-197, March 1978.60-A191 1978

Bangil, M.; Soubrier, M.; Dubost, J. J.; Rami, S.; Carcanagues, Y.; Ristori, J. M.; and Bussiere, J. L.: Subchondral insufficiency fracture of the femoral head. Rev. Rheumat.,63: 859-861, 1996.63859 1996

Bullough, P. G.: Injury and repair: bone. In Atlas of Orthopaedic Pathology. Ed. 3, pp. 81-106. New York, Mosby-Wolfe, 1997.

Hagino, H.; Okano, T.; Teshima, R.; Nishi, T.; and Yamamoto, K.: Insufficiency fracture of the femoral head in patients with severe osteoporosis - report of 2 cases. Acta Orthop. Scandinavica,70: 87-89, 1999.7087 1999

Jacobs, M. A.; Loeb, P. E.; and Hungerford, D. S.: Core decompression of the distal femur for avascular necrosis of the knee. J. Bone and Joint Surg.,71-B(4): 583-587, 1989.71-B(4)583 1989

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TABLE I: Clinicopathological Characteristics of the Fourteen Patients

Case	Gender, Age (yrs.)	Side	Height (m)	Weight (kg)	Body-Mass Index*(kg/m²)	Steroid Intake	Alcohol Abuse	Radiographic Stage	Degree of Pain	Treatment†	Time Between Onset of Pain and Op. (mos.)	Associated Conditions
Group I (subchondral fracture)
1	F, 83	L	1.45	73	34.7	No	No	2	Severe	Anti-inflammatory drugs, assistive device	3
2	M, 61	R	1.77	82	26.2	No	No	2	Severe	Anti-inflammatory drugs, assistive device	6	Bilateral femoral neck fracture 3 mos. after op.
3	M, 72	R	1.73	109	36.4	No	No	2	Moderate	Anti-inflammatory drugs, assistive device	7
Group II (subchondral fracture with osteonecrosis)
4	F, 69	R	1.60	77	30.1	No	No	3	Severe	Anti-inflammatory drugs	4
5	F, 70	R	1.57	68	27.6	No	No	3	Severe	Anti-inflammatory drugs, assistive device	4	Right femoral neck fracture 1 yr. after op.
6	F, 73	R	1.62	73	27.8	No	No	3	Severe	Anti-inflammatory drugs, assistive device	5
7	F, 75	L	1.50	74	32.9	No	No	3	Moderate	Anti-inflammatory drugs, assistive device	5
8	F, 72	L	1.53	60	25.6	No	No	3	Moderate	Anti-inflammatory drugs	12
9	M, 71	L	1.68	75	26.6	No	No	3	Moderate	Anti-inflammatory drugs	8
Group III (indeterminate)
10	F, 76	L	1.52	84	36.4	No	No	3	Severe	Anti-inflammatory drugs, assistive device	8
11	F, 88	R	1.60	56	21.9	No	No	3	Moderate	Anti-inflammatory drugs, assistive device	5
12	M, 86	R	1.68	68	24.1	No	No	3	Moderate	Anti-inflammatory drugs, assistive device	24
13	M, 70	R	1.78	79	24.9	No‡	No	4	Moderate	Anti-inflammatory drugs, corticosteroid injection after onset of pain	10
14	M, 59	L	1.75	81	26.4	No	No	4	Moderate	Anti-inflammatory drugs	13

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TABLE I: Clinicopathological Characteristics of the Fourteen Patients

Case	Gender, Age (yrs.)	Side	Height (m)	Weight (kg)	Body-Mass Index*(kg/m²)	Steroid Intake	Alcohol Abuse	Radiographic Stage	Degree of Pain	Treatment†	Time Between Onset of Pain and Op. (mos.)	Associated Conditions
Group I (subchondral fracture)
1	F, 83	L	1.45	73	34.7	No	No	2	Severe	Anti-inflammatory drugs, assistive device	3
2	M, 61	R	1.77	82	26.2	No	No	2	Severe	Anti-inflammatory drugs, assistive device	6	Bilateral femoral neck fracture 3 mos. after op.
3	M, 72	R	1.73	109	36.4	No	No	2	Moderate	Anti-inflammatory drugs, assistive device	7
Group II (subchondral fracture with osteonecrosis)
4	F, 69	R	1.60	77	30.1	No	No	3	Severe	Anti-inflammatory drugs	4
5	F, 70	R	1.57	68	27.6	No	No	3	Severe	Anti-inflammatory drugs, assistive device	4	Right femoral neck fracture 1 yr. after op.
6	F, 73	R	1.62	73	27.8	No	No	3	Severe	Anti-inflammatory drugs, assistive device	5
7	F, 75	L	1.50	74	32.9	No	No	3	Moderate	Anti-inflammatory drugs, assistive device	5
8	F, 72	L	1.53	60	25.6	No	No	3	Moderate	Anti-inflammatory drugs	12
9	M, 71	L	1.68	75	26.6	No	No	3	Moderate	Anti-inflammatory drugs	8
Group III (indeterminate)
10	F, 76	L	1.52	84	36.4	No	No	3	Severe	Anti-inflammatory drugs, assistive device	8
11	F, 88	R	1.60	56	21.9	No	No	3	Moderate	Anti-inflammatory drugs, assistive device	5
12	M, 86	R	1.68	68	24.1	No	No	3	Moderate	Anti-inflammatory drugs, assistive device	24
13	M, 70	R	1.78	79	24.9	No‡	No	4	Moderate	Anti-inflammatory drugs, corticosteroid injection after onset of pain	10
14	M, 59	L	1.75	81	26.4	No	No	4	Moderate	Anti-inflammatory drugs	13