The pathomechanical etiology of post-traumatic osteoarthritis following intraarticular fractures

Abstract

Many intra-articular fracture patients eventually experience significant functional deficits, pain, and stiffness from post-traumatic osteoarthritis (PTOA). Over the last several decades, continued refinement of surgical reconstruction techniques has failed to markedly improve patient outcomes. New treatment paradigms are needed - ideally, bio/pharmaceutical. Progress in that direction has been impeded because the pathomechanical etiology of PTOA development is poorly understood. In particular, the relative roles and pathomechanisms of acute joint injury (from the initial trauma) versus chronic contact stress elevation (from residual incongruity) are unknown, primarily because there have been no objective methods for reliably quantifying either of these insult entities. Over the past decade, novel enabling technologies have been developed that provide objective biomechanical indices of injury severity and of chronic contact stress challenge to fractured joint surfaces. The severity of the initial joint injury is indexed primarily on the basis of the energy released in fracture, obtained from validated digital image analysis of CT scans. Chronic contact stress elevations are indexed by patient-specific finite element stress analysis, using models derived from post-reduction CT scans. These new measures, conceived in the laboratory, have been taken through the stage of validation, and then have been applied in studies of intra-articular fracture patients, to relate these biomechanical indices of cartilage insult to the incidence and severity of PTOA This body of work has provided a novel framework for developing and testing new approaches to forestall PTOA following intra-articular fractures.

Figure 1

These radiographs illustrate the tibial plafond fracture severity spectrum. Simple intra-articular fractures result from low energy impacts (left). As energy increases, the fractures become more complex, with greater comminution (right).

Figure 2

Bone perimeters (matched intact & fractured), plotted along the length of the distal tibia, show how the fracture energy measure is calculated. Inset: CT slice from fracture case, with identified tibia bone fragment edges.

Figure 3

Controlled fracture experiments performed in bone showed a highly linear relationship between CT-inferred fracture energy and the physically measured energy absorbed in fracture. Inset: The data also showed that fragment sizes correlated with fracture energy, shifting from predominantly larger fragments at lower energies to smaller fragments at higher energies. [Adapted with permission from Anderson DD et al. Quantifying tibial plafond fracture severity: absorbed energy and fragment displacement agree with clinical rank ordering. J Orthop Res. 26:1046-52, 2008.40]

Figure 4

Depiction of the fragment displacement/dispersion metric calculation. [Adapted with permission from Anderson DD et al. Quantifying tibial plafond fracture severity: absorbed energy and fragment displacement agree with clinical rank ordering. J Orthop Res. 26:1046-52, 2008.40]

Figure 5

(a) Standard, unsegmented rendering from radiology workstation: visually informative, but with no active functionality. Following segmentation, (b) individual fragments (49 of them in this case) may be readily, and independently studied (transparent surface is intact contralateral, mirrored and aliened Droximallv).

Figure 6

Agreement between injury severity rankings and CT-based metrics. The graphs compare the rank ordering of rater 1 versus that of raters 2 and 3, and of the individual CT-based metrics. Concordance values are enclosed in parentheses following the rater/metric. [Adapted with permission from Anderson DD et al. Quantifying tibial plafond fracture severity: absorbed energy and fragment displacement agree with clinical rank ordering. J Orthop Res. 26:1046-52, 2008.40]

Figure 7

(a) The voxellated segmentation of the relaxed posture intact ankle is smoothed into (b) a geometric model of the bone surfaces, suit-able for manipulation and FE solution, (c) The re-orientations required to bring the as-segmented ankle into a neutral apposition are shown, with the partially translucent image showing the original state of the talus. [Adapted with permission from Anderson DD et al. Intra-articular contact stress distributions at the ankle throughout stance phase-patient-specific finite element analysis as a metric of degeneration propensity. Biomech Model Mechanobiol. 5:82-9, 2006.52]

Figure 8

(A) Schematic of ankle loading, showing an ankle speci-men, the contact stress sensor, and the fixture. (B) Radiographs, and virtual representation of the K-wires for registration. (C) Pinal regis-tration of FE to experimental results. [Adapted with permission from Anderson DD et al. Physical validation of a patient-specific contact finite element model of the ankle. J Biomech. 40:1662-9, 2007.54]

Figure 9

Inferior view of tibia, overlaid with spatially aligned Tekscan pressure results (left) and FE results (right) for each validation ankle. [Reprinted with permission from Anderson DD et al. Physical validation of a patient-specific contact finite element model of the ankle. J Biomech. 40:1662-9, 2007.54]

Figure 10

An antero-superior (subchondral) view showing the contact stress distributions computed for the 13 instants of the stance phase of gait for an intact ankle. [Reprinted with permission from Li W et al. Patient-specific finite element analysis of chronic contact stress exposure after intra articular fracture of the tibial plafond. J Orthop Res. 26:1039-45, 2008.61]

Figure 11

FE-computed contact stress exposure distributions for the 11 paired intact and fractured (post-reduction) ankles, for a single gait cycle. [Adapted with permission from Li W et al. Patient-specific finite element analysis of chronic contact stress exposure after intraarticular fracture of the tibial plafond. J Orthop Res. 26:1039-45, 2008.61]

Figure 12

A combined severity score including fracture energy and articular comminution predicted 70% of the variation in KL arthrosis grade at two-year follow-up. [Adapted with permission from Thomas TP et al. Objective CT-based metrics of articular fracture severity to assess risk for posttraumatic osteoarthritis. J Orthop Trauma. 24:764-9, 2010.67]

Figure 13

The CT-based severity metric successfully discriminated between cases that developed PTOA and those that did not, in a threshold like manner

Figure 14

The % of contact area engaged above selected contact stress levels shows that in the ankles that went on to develop PTOA, a much larger percentage of the post-fracture cartilage surface was subject to high levels of contact stress (dark columns). The inset shows contact stress distributions for a representative pairing of a fractured ankle with its intact contralateral. [Reprinted with permission from Anderson DD et al. Is elevated contact stress predictive of post-traumatic osteoarthritis for imprecisely reduced tibial plafond fractures? J Orthop Res. 29:33-9, 2011.68]

Figure 15

The relative proportion of articular surface area experiencing supra-threshold contact stress, associated with a stress-time exposure in excess of 3MPa-s, agreed closely with the two-year KL score, and was perfectly predictive of OA status. [Reprinted with permission from Anderson DD et al. Is elevated contact stress predictive of post-traumatic osteoarthritis for imprecisely reduced tibial plafond fractures? J Orthop Res. 29:33-9, 2011.68]

Figure 16

Focal regions of decreased cartilage thickness showed a correspondence with elevated chronic contact stress at two-years follow-up.

Figure 17

(A) A tibial plafond fracture case is first segmented from CT data. (B) The intact template is registered to the fractured limb's proximal base, and me fragment native surfaces are aligned to me template, starting with the articular block.