Rationale and Objectives
Subchondral insufficiency fractures (SIF), previously termed spontaneous osteonecrosis of the knee, are marked by a sudden onset of severe pain. Other than the size of the lesion, prediction for progression to joint replacement is difficult. The objective was to determine if quantitative analysis of bone texture using digital tomosynthesis imaging would be useful in predicting more rapid progression to joint replacement.
Materials and Methods
Tomosynthesis studies of 30 knees with documented SIF were quantified by fractal, mean intercept length (MIL), and line fraction deviation analyses. Fractal dimension, lacunarity, MIL, and line fraction deviation variables measured from these analyses were then correlated to short interval progression to joint replacement surgery.
Results
Higher odds for joint replacement were related to higher values of the standard deviation of slope lacunarity and to morphometric measures (eg, MIL).
Conclusions
Using digital tomosynthesis images for bone texture assessment may help distinguish condylar bone response in SIF, potentially acting as a clinically relevant predictive tool. In the future, contrasting SIF to the more gradual long-term process of osteoarthritis, there may be a better understanding of the different mechanisms for the two conditions.
Introduction
Subchondral Insufficiency Fractures (SIF)
The term spontaneous osteonecrosis of the knee was first described as a finding in osteoarthritis . The condition usually presents with a sudden onset of severe, acute, unilateral knee pain, mostly in women more than 50 years old. It has been renamed as there is usually no history of trauma and the majority of patients have no risk factors for osteonecrosis . Many patients have a very painful course that can be followed by resolution of symptoms over months. Progressive collapse can occur and may lead to early surgical interventions, including joint replacement . Insufficiency fractures have been linked with osteoarthritis . In broader use, the term insufficiency fracture has been defined as a fracture that occurs due to the inability of the bone to support normal loads . The term implies that an insufficiency fracture would occur only in individuals with poor bone quality or mass. However, review of a larger series of patients with SIF reveals a systemic bone mass that is above normal for age . This is in contrast to patients with early osteoarthritis who appear to have decreased bone perfusion and osteocyte activity in the femoral condyle . Histology of SIF has shown that in six of eight cases, there was no osteonecrosis, and the only areas of osteonecrosis in the remaining two were in regions of bone collapse . Four stages have been described comparing plain radiographs to histology . The radiographic stage ranged from no abnormality to a lucent area surrounded by sclerotic bone with osteoarthritic changes. Histology ranged from areas of granulation with fracture healing and no osteonecrosis, to cases of focal necrosis or complete separation. These patterns are very different from the patterns seen in secondary osteonecrosis.
Clinical Imaging in SIF
When radiographs are obtained within weeks of the onset of severe pain, they may appear normal with little or no evidence of osteoarthritic changes . The biologic activity within and outside of these lesions has been documented with fluorine-18 positron emission tomography . The overall size of the lesion correlates to what is seen on magnetic resonance imaging (MRI). The T1 or proton density-weighted image typically has a curvilinear area of decreased signal that parallels the articular margin of the subchondral bone of the proximal tibial plateaus and femoral condyles . All lesions have direct connection to the articular subchondral bone. In one review, the average surface area was 431 mm 2 (standard deviation [SD]: 218 mm 2 ) (range: 210–1025 mm 2 ) and the volume was 4.8 cm 3 (SD: 3.1 cm 3 ), placing the diameter between 0.5 and 2 cm . There are a variety of MRI bone marrow lesions seen in osteoarthritis . However, none of these have the clear line of demarcation that is seen in SIF ( Fig 1 ). Albeit the size of the lesion is predictive for progression to total knee replacement , smaller lesions can progress to rapid articular cartilage changes. Given that bone changes are not apparent in conventional radiology, we have not had an imaging tool that can better predict clinical outcome or help define the biologic process.
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Tomosynthesis to Evaluate SIF
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Materials and Methods
Patient Population, Demographics, and Clinical Imaging
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Tomosynthesis Image Analyses
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Table 1
Measured Texture Parameters from DTS Images and Definitions
DTS Variable Description Slice FD Fractal dimension: measure of complexity in gray-level texture λ Lacunarity: measure of heterogeneity in the size of holes in gray-level texture S λ Slope lacunarity: rate of change in λ with size scale LFD.Av LFD average: measure of average orientation in all directions in gray-level texture LFD.SD LFD standard deviation: measure of variability in the orientation of gray-level texture LFD.Max LFD maximum: measure of maximum orientation in gray-level texture LFD.DA LFD degree of anisotropy: measure of anisotropy in gray-level texture MIL.Av MIL average: measure of average feature size in all directions of binarized texture MIL.SD MIL standard deviation: measure of variability of feature size in all directions of binarized texture MIL.Max MIL maximum: measure of maximum feature size in binarized texture MIL.DA MIL degree of anisotropy: measure of anisotropy in binarized texture Stack Av(FD) Interslice average of respective fractal parameters, representing a volume average Av(λ) Av(S λ ) Av(LFD.Av) Interslice average of respective LFD parameters, representing a volume average Av(LFD.SD) Av(LFD.Max) Av(LFD.DA) Av(MIL.Av) Interslice average of respective MIL parameters, representing a volume average Av(MIL.SD) Av(MIL.Max) Av(MIL.DA) SD(FD) Interslice heterogeneity of respective fractal parameters, representing plane to plane variation in the respective parameter SD(λ) SD(S λ ) SD(LFD.Av) Interslice heterogeneity of respective LFD parameters, representing plane to plane variation in the respective parameter SD(LFD.SD) SD(LFD.Max) SD(LFD.DA) SD(MIL.Av) Interslice heterogeneity of respective MIL parameters, representing plane to plane variation in the respective parameter SD(MIL.SD) SD(MIL.Max) SD(MIL.DA)
Parameters from fractal, mean intercept length (MIL), and line fraction deviation (LFD) analyses were measured both in whole stack (from which stack average [Av] and standard deviation [SD] were calculated) and a central slice.
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Statistical Analyses
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Results
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Table 2
Summary of the Associations Between Digital Tomosynthesis-derived Bone Texture and Lesion Site, After Accounting for Significant Anatomic Variation (Marginal Mean ± Standard Error) \*
DTS Variable At Lesion Away From Lesion_P_ Bone † P Site ‡ P Lesion § λ 0.064 ± 0.005 0.073 ± 0.003 <0.02 <0.0001 0.061 Av(λ)0.068 ± 0.0030.073 ± 0.002 NS <0.0001<0.05 SD(λ) 0.0097 ± 0.0015 0.0123 ± 0.0007 <0.02 <0.0001 0.053 SD(S λ ) 0.0043 ± 0.0004 0.0050 ± 0.0002 <0.0001 <0.0001 0.080 Av(LFD.DA)1.88 ± 0.192.30 ± 0.05 <0.0001 <0.0001<0.04 SD(LFD.DA)0.35 ± 0.090.58 ± 0.03 NS <0.004<0.02 SD(MIL.Max)−0.039 ± 0.1390.315 ± 0.049 NS <0.0003<0.02 SD(MIL.Av)−0.057 ± 0.1260.268 ± 0.042 NS <0.001<0.02 SD(MIL.DA)0.069 ± 0.0060.057 ± 0.003 NS <0.0001<0.02 SD(MIL.SD) 0.027 ± 0.020 0.062 ± 0.007 NS <0.0001 0.075
For ease of viewing, table entries are bolded if the effect of primary interest ( P lesion) is statistically significant.
λ, mean lacunarity; Av, mean; DA, degree of anisotropy; DTS, digital tomosynthesis; LFD, line fraction deviation; Max, maximum; MIL, mean intercept length; NS, not significant; SD, standard deviation; S λ , slope lacunarity.
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Table 3
Summary of Digital Tomosynthesis-derived Bone Texture Variables Associated with Odds for TKA (Mean ± Standard Error) \*
DTS Variable Resolved TKA_P_ for Odds SD(FD) ( at lesion ) 0.0093 ± 0.0005 0.0118 ± 0.0010 0.076 S λ 0.052 ± 0.0010.055 ± 0.002<0.03 Av(S λ ) 0.052 ± 0.001 0.054 ± 0.002 0.077 SD(λ) 0.012 ± 0.001 0.010 ± 0.001 0.092 SD(S λ ) 0.0050 ± 0.0003 0.0044 ± 0.0003 0.068 SD(LFD.SD) 0.00069 ± 0.00012 0.00045 ± 0.00006 0.064 SD(MIL.Av) 0.255 ± 0.046 0.123 ± 0.014 0.081 SD(MIL.DA) 0.056 ± 0.003 0.070 ± 0.007 0.094
For ease of viewing, the statistically significant result is bolded.
λ, mean lacunarity; Av, mean; DA, degree of anisotropy; DTS, digital tomosynthesis; FD, fractal dimension; LFD, line fraction deviation; MIL, mean intercept length; SD, standard deviation; S λ , slope lacunarity; TKA, total knee arthroplasty.
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Discussion
Specific Findings and the Rare Incidence of SIF
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Interpretation of the Findings
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Study Deficiencies
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Conclusions
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Acknowledgment
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