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Spontaneous Osteonecrosis of the Knee

Zaremski, Jason L. MD, CAQSM, FACSM; Vincent, Kevin R. MD, PhD, CAQSM, FACSM

doi: 10.1249/JSR.0000000000000271
Clinical Pearls
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Department of Orthopaedics and Rehabilitation, Divisions of PM&R, Sports Medicine, and Research, University of Florida College of Medicine, Gainesville, FL

Address for correspondence: Jason L. Zaremski, MD, CAQSM, FACSM, Department of Orthopaedics and Rehabilitation, Divisions of PM&R, Sports Medicine, and Research, 3450 Hull Road, University of Florida, Gainesville, FL 32607; E-mail: zaremjl@ortho.ufl.edu.

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Background

Spontaneous osteonecrosis of the knee (SONK), first described in 1968 (1), is an ailment that typically affects the medial femoral condyle (94%) (3,7), although it also has been described to affect the lateral femoral condyle, tibia, and patella (4,8,11).

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Pathophysiology

  • Likely due to subchondral insufficiency fractures in osteopenic bone without evidence of necrosis (2,16).
  • Edema from insufficiency fracture can lead to secondary focal ischemia and necrosis.
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Typical Clinical Presentation (7,12,15)

  • Women who are 50 to 60 years or older
  • Typically active, exercises, and unilateral
  • History of osteoporosis/osteopenia
  • Acute onset medial-sided knee pain without precipitating trauma
  • Exacerbated with weight bearing
  • The most common physical examination finding is severe pain with palpation over the medial femoral condyle (10).
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Important Historical Information

  • History of surgery and/or trauma to the affected knee
  • Disorders of bone
  • Prior meniscal injury
  • Pain usually worse at night and with weight bearing
  • Prior injectable and/or oral steroid usage
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Imaging

Four view radiographs (anterior-posterior, tunnel view, sunrise, and lateral) should be obtained. There will be flattening of the respective condyle and/or radiolucencies if SONK is in the late stages; otherwise, radiographs will be normal in the early stages. Magnetic resonance imaging (MRI) is the imaging modality of choice if SONK is suspected (Fig. 1) (9). Bone scans also may show SONK (Fig. 2) but are not as sensitive as MRI (7).

Figure 1

Figure 1

Figure 2

Figure 2

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Characteristics of SONK-MRI

  • Bone marrow edema out of proportion to osteoarthritic changes (Fig. 1)
  • Subchondral crescent linear focus on T1 and potentially T2 sequences
  • Focal epiphyseal contour depression
  • Subchondral low signal (5,9)
  • Association with meniscal tears, specifically radial and root tears (14)
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Differential Diagnosis

  • SONK (subchondral insufficiency fracture)
  • Bone contusion
  • Transient osteoporosis of bone
  • Osteochondritis dissecans
  • Exacerbation of osteoarthritis
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Treatment

Treatment and prognosis is dependent on the extent of T2 subchondral signal and size of subchondral lesion (7,9,17). If detected early and the subchondral lesion is small (one algorithm proposes less than 3.5 cm2) (7), nonsurgical management is appropriate. Nonsurgical management includes non-weight bearing versus protected weight bearing with a medial off-loader knee brace, analgesics, nonsteroidal anti-inflammatory drugs as needed, and potentially bisphosphonates (6,17). Weight bearing status is dependent upon symptoms. If the lesion is larger (>50% of the femoral condyle or lesion size is greater than 5 cm), patients do not improve nonoperatively, or radiographically after 3 months, there is an increased risk of collapse and surgical referral is indicated (7,10,13).

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Pearl

If there is pain out of proportion to the examination over the medial femoral condyle without trauma and normal radiographs (except for osteoarthritic changes), one should have a low threshold to order an MRI to detect early onset of SONK.

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References

1. Ahlbäck S, Bauer GC, Bohne WH. Spontaneous osteonecrosis of the knee. Arthritis Rheum. 1968; 11:705–33.
2. Akamatsu Y, Mitsugi N, Hayashi T, et al. Low bone mineral density is associated with the onset of spontaneous osteonecrosis of the knee. Acta Orthop. 2012; 83:249–55.
3. al-Rowaih A, Björkengren A, Egund N, et al. Size of osteonecrosis of the knee. Clin. Orthop. Relat. Res. 1993; 287:68–75.
4. Ecker ML, Lotke PA. Spontaneous osteonecrosis of the knee. J. Am. Acad. Orthop. Surg. 1994; 2:173–8.
5. Gil HC, Levine SM, Zoga AC. MRI findings in the subchondral bone marrow: a discussion of conditions including transient osteoporosis, transient bone marrow edema syndrome, SONK, and shifting bone marrow edema of the knee. Semin. Musculoskelet. Radiol. 2006; 10:177–86.
6. Jureus J, Lindstrand A, Geijer M, et al. Treatment of spontaneous osteonecrosis of the knee (SONK) by a bisphosphonate. Acta Orthop. 2012; 83:511–4.
7. Karim AR, Cherian JJ, Jauregui JJ, et al. Osteonecrosis of the knee: review. Ann. Transl. Med. 2015; 3:6.
8. LaPrade RF, Noffsinger MA. Idiopathic osteonecrosis of the patella: an unusual cause of pain in the knee. A case report. J. Bone Joint Surg. Am. 1990; 72:1414–8.
9. Lecouvet FE, van de Berg BC, Maldague BE, et al. Early irreversible osteonecrosis versus transient lesions of the femoral condyles: prognostic value of subchondral bone and marrow changes on MR imaging. AJR Am. J. Roentgenol. 1998; 170:71–7.
10. Lotke PA, Abend JA, Ecker ML. The treatment of osteonecrosis of the medial femoral condyle. Clin. Orthop. Relat. Res. 1982; 171:109–16.
11. Lotke PA, Nelson CL, Lonner JH. Spontaneous osteonecrosis of the knee: tibial plateaus. Orthop. Clin. North Am. 2004; 35:365–70.
12. Mears SC, McCarthy EF, Jones LC, et al. Characterization and pathological characteristics of spontaneous osteonecrosis of the knee. Iowa Orthop. J. 2009; 29:38–42.
13. Mont MA, Marker DR, Zywiel MG, et al. Osteonecrosis of the knee and related conditions. J. Am. Acad. Orthop. Surg. 2011; 19:482–94.
14. Omar I, Zoga AC, Morrison WB. Types of meniscal tears present in association with SONK (spontaneous osteonecrosis of the knee). Presented at the annual proceedings of the American Roentgen Ray Society; 2006; Vancouver (BC).
15. Pape D, Seil R, Fritsch E, et al. Prevalence of spontaneous osteonecrosis of the medial femoral condyle in elderly patients. Knee Surg. Sports Traumatol. Arthrosc. 2002; 10:233–40.
16. Yamamoto T, Bullough PG. Spontaneous osteonecrosis of the knee: the result of subchondral insufficiency fracture. J. Bone Joint Surg. Am. 2000; 82:858–66.
17. Yates PJ, Calder JD, Stranks GJ, et al. Early MRI diagnosis and non-surgical management of spontaneous osteonecrosis of the knee. Knee. 2007; 14:112–6.
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