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Novel biomechanical methods have been developed to objectively measure acute fracture severity (from inter-fragmentary surface area) and chronic contact stress challenge (from patient-specific finite element analysis) in articular fractures. These new methods help clarify the pathomechanics of the development of post-traumatic osteoarthritis, and can contribute directly to the clinical care of patients. In this manuscript, the value of these two new measures is demonstrated in three illustrative tibial plafond fracture cases, in which both metrics are correlated with cartilage status and with patient outcomes at a minimum of two years after injury. These clinical cases demonstrate the utility of new biomechanical variables to advance clinical research and patient care, by providing a basis to predict outcome and select treatment.
Fractures involving the articular surface of weight-bearing joints often result in post-traumatic osteoarthritis (PTOA), chronic pain, and subsequent poor joint function.1,2 Although the fundamental mechanisms that lead to PTOA are not well understood, studies have shown that the degree of articular fracture severity and of post-fracture joint incongruity both correlate with the development of PTOA.2-7 Unfortunately, elucidating the interactions between these important mechanical factors has been hampered by a lack of methods to objectively measure them. Traditionally, fracture severity and articular malreduction are subjectively assessed on radiographs, using surgeon judgment alone via categorical classifications. Since these methods are known to have poor inter-observer reliability5,8 and to provide inadequate precision for measuring articular surface step-offs,9,10 techniques that are more objective and quantitative are needed to assess the mechanical risk factors for developing PTOA.
Novel objective methods have been recently developed to quantify acute injury severity,3,11,12 post-reduction articular contact stress,13,14 and cartilage thickness at two years post-injury.15 These techniques have separately correlated with patient outcomes.3,11,12 However, in addition to independent analysis, these metrics need to be evaluated in concert to better understand factors contributing to cartilage loss and to the development of PTOA.
Between 2001 and 2005, 36 patients with tibial plafond fractures were uniformly treated by the same surgical team, with joint-spanning external fixation and limited approaches to reduce and fix the articular surface with screws. After receiving Institutional Review Board (IRB) approval, 22 of these patients were consented for study, with 11 of them completing all phases of the evaluation. Their acute fracture severity was quantitatively assessed by measuring fracture energy, a pre-reduction CT-based metric that is calculated from inter-fragmentary surface area and bone density information. Their chronic cartilage contact stress was assessed using voxel-based finite element analyses derived from post-reduction CT scans.
The purpose of this manuscript is to present three illustrative cases from this series of patients, linking the relative effect of acute fracture severity and chronically elevated joint contact stress on cartilage thickness with clinical outcome. Three-dimensional cartilage thickness information was obtained with double-contrast multide-tector CT (MDCT) scans at greater than two years post-injury. The independent variables were the metrics of fracture severity and of chronic contact stress challenge.
A 26 year old male fell from a height of 16 feet and sustained an open right tibial plafond fracture. Radiographs showed a highly comminuted fracture involving the articular surface and metaphysis, extending into the distal diaphysis, classified as OTA type 43C-3 (Figures 1a, b). The energy of fracture, computed from CT inter-fragmentary area, was 24.4 Joules (Figure 1c). The fracture was initially managed with irrigation and debridement, and stabilized with a joint-spanning external fixator. An anteroposterior (AP) radiograph (Figure 2a), and a coronal CT slice (Figure 2b) demonstrate the degree of joint comminution. Four days following injury, the articular surface was reduced through a limited anterolateral approach, and was fixed with four partially threaded cancellous screws. A post-operative AP radiograph (Figure 3a) shows the alignment after this procedure, and a post-operative coronal CT slice (Figure 3b) and volumetric rendering (Figure 3c) reveal residual joint surface incongruity.
At 30 months after injury, radiographs show a healed fracture (Figures 4a, b). The peak contact stress exposure, determined from FEA was 7.3 MPa-s, versus 5.8 MPa-s for the uninjured contralateral ankle (Figure 4c). An anterior osteophyte had developed and there was loss of joint space anteriorly. The Kellgren-Lawrence OA grade was 4, and the Ankle Osteoarthritis Score (AOS) was 63/90 for pain and 65/90 for disability (higher scores indicate more pain and disability).16,17 The cartilage loss is better visualized on the double-contrast MDCT scans (Figures 4d, e), which showed cartilage thinning and absence of cartilage over the anterior and posterolateral areas of the ankle joint.
A 41 year old male fell from a height of 10 feet and sustained a left tibial plafond fracture. His radiographs (Figures 5a, b) showed a partial articular fracture of the tibial plafond classified as 43B-2. The fracture energy was 4.8 Joules (Figure 5c). An anterolateral fracture fragment was clearly seen on CT (Figure 5d). The fracture was treated with a joint-spanning external fixator, closed reduction of the anterolateral fragment, and percutaneous screw fixation. Post-operative AP (Figure 6a) and lateral (Figure 6b) radiographs, along with axial CT slice (Figure 6c) and volumetric rendering (Figure 6d), suggest some residual displacement of the anterolateral fragment.
Twenty-seven months after injury, the lateral radiograph shows joint space narrowing anteriorly, with probable osteophyte formation (Figure 7a). The Kellgren-Lawrence grade was 2, and the AOS ankle score was 34/90 for pain and 30/90 for disability. Anterior cartilage loss and subchondral cysts were seen on the double-contrast MDCT scan (Figure 7b). The peak contact stress exposure was 6.5 MPa-s, versus 3.7 MPa-s for the uninjured contralateral ankle (Figure 7c).
A 41 year old female sustained an isolated right tibial plafond fracture in a motor vehicle accident. Initial radiographs revealed an oblique distal tibial fracture with a single intra-articular fracture line, classified as 43C-2 (Figures 8a, b). The CT scan indicated a fracture energy of 9.9 Joules (Figure 8c), and revealed primarily rotational displacement of the main articular fragment (Figure 8d). Two days following the injury, her fracture was treated with a spanning external fixator, and the joint was percutaneously reduced and fixed with partially threaded screws. Post-operative radiographs and CT show the reduction and joint alignment achieved during this surgery (Figures 9a-e).
Twenty-four months after injury, AP and lateral radiographs reveal a healed fracture with a well-aligned articular surface and well-preserved joint space (Figures 10a, b). The Kellgren-Lawrence grade was zero, and the AOS ankle score was 22/90 for pain and 24/90 for disability. A double-contrast MDCT scan showed an even distribution of contrast with preserved cartilage thickness on both the bottom of the tibia and the top of the talus (Figures 10c, d). The peak contact stress exposure was 5.1 MPa-s, essentially the same as for the uninjured contralateral ankle (5.6 MPa-s, Figure 10e).
With 24.4 Joules of fracture energy, this case had the greatest acute fracture severity among the 22 fractures studied. In Figure 11, the fracture-liberated surface area (mm2) is plotted along the length of the fractured tibia as a solid black line. The inter-fragmentary surface area is high throughout the distal third of the tibia, but it is particularly high directly adjacent to the articular surface. The post-operative CT served as the source material for computational (finite element) analysis of the joint contact stress for this ankle. Compared to the intact ankle, there was significantly elevated contact stress challenge in the fractured ankle (Figure 12). The peak contact stress exposure for this patient was 7.3 MPa-s, among the highest of the 22 fractures studied.
Of the 22 cases studied, this was the least severe case, with a fracture energy of only 4.8 Joules. In Figure 11 this patient's inter-fragmentary surface area is shown as the solid gray line. Finite element analysis based on the post-operative CT scan illustrates the left and right ankle contact stress distributions (Figure 12). Elevated contact stress challenge was seen over the anterior distal tibia on the left side, compared to the intact right ankle. The peak contact stress exposure for this patient was 6.5 MPa-s, which was approximately at the median of the 22 fractures studied.
The total energy of this fracture calculated from the pre-reduction CT was 9.9 Joules. The dotted black line in Figure 11 shows the liberated surface area (mm2) along the length of the fractured tibia. Finite element analysis from the post-operative CT illustrates that the chronic cartilage contact stress challenge for the fractured ankle and the intact ankle (Figure 12) was nearly equivalent. The peak contact stress exposure for the injured ankle was 5.1 MPa-s, which was the lowest among the 22 fractures studied.
The ability to reliably predict which patients will develop disabling PTOA following intra-articular fractures of weight-bearing joints has long eluded orthopaedic surgeons, since the predisposing mechanical and biological mechanisms are poorly understood and have not been amenable to rigorous quantitative analysis. Over the last several years, we have developed methods which can be used to objectively quantify acute fracture severity,3,11,12 post-reduction chronic joint contact stress challenge,13,14 and resulting cartilage thickness changes.15 These quantitative metrics serve as a platform to better understand the relative contribution of mechanical risk factors on cartilage loss, and on the development of PTOA. The three cases presented demonstrate the application of these measurements in individual patients with very different tibial plafond fracture challenges, with a view toward understanding the relative effect of mechanical factors on outcome at a minimum of two years after injury.
Injury severity influences clinical decision-making when treating intra-articular fractures. Injury severity is often assessed on plain radiographs by labeling them as “high” or “low” energy. Since energy is a mechanical unit that can be quantified, those qualitative terms can be objectively measured with CT-based methods. Such techniques have been effective; studies have shown fracture energy to generally correlate with the opinions of experienced clinicians.11 For articular fractures of the distal tibia, correlating this metric with outcome indicates a threshold for energy of injury, above which PTOA is much more likely to develop. This is illustrated in Case 1: a highly comminuted tibial plafond fracture with a very high energy of injury (24.4 J) distributed along the length of the distal tibia, and particularly concentrated adjacent to the articular surface. Although finite element analysis showed significant residual increased contact stress challenge, the early onset of severe PTOA was almost certainly a direct result of the severity of the initial insult. The blunt impact, shattering the articular surface, likely led to widespread cartilage necrosis that was incompatible with preserving a functioning cartilage surface.
A basic principle of treating articular fractures is that to optimize the chances for a favorable outcome, the articular surface must be reduced to an extent that deleteriously elevated joint contact stress is minimized. Directly quantifying joint contact stress using finite element analysis (based on post-reduction CT scans) has shown that there apparently is a threshold above which the cartilage cannot tolerate these abnormal stresses over time.13
Case 2 illustrates how chronic joint contact stress challenge may be the primary factor leading to cartilage injury and PTOA. In contrast to Case 1, the energy of injury was only 4.8 J, indicating that the cartilage in this joint probably survived the acute injury. However, finite element analysis showed increased contact stress challenge over the anterior aspect of the injured ankle, in the area of imperfect articular reduction. At 27 months after injury, double-contrast MDCT scans demonstrated cartilage thinning in the same anterolateral portion of the joint. Increased contact stress exposure over time from articular malreduction led to a loss of articular cartilage in this case. The treatment failed to save a potentially salvageable articular surface.
Unlike the first two cases, which were at the extremes of fracture severity, the energy of injury of Case 3 was at an intermediate level of 9.9 J. At this level of injury it is possible that a perfect articular reduction with minimal increases in chronic joint contact loading was required for the moderately injured joint cartilage to survive. Indeed, post-reduction finite element analysis showed no increased contact stress compared to the opposite side, and a favorable outcome with well-preserved cartilage distribution at 24 months after injury was the result. This fracture had mid-level energy and low postoperative contact stress, and the cartilage was preserved throughout the joint.
These illustrative cases (from a larger series) demonstrate the type of analyses that have become possible through quantitative assessment of critical mechanical variables. More fractures, in larger multi-center series and with long-term follow-up, need to be studied to move the treatment further toward a more objective science, where management decisions are in part guided by quantitative assessment of the disease process, rather than the current practice, where they are based solely on surgeon opinion.
In summary, three illustrative cases have been presented from a larger series of patients with tibial plafond fractures, where injury severity and post-treatment contact stress have been objectively measured and correlated with a minimum two-year outcome. The relative role of each of these mechanical factors is discussed in the context of both acute injury severity and chronic contact stress challenge interacting to influence the development of PTOA. These cases illustrate the potential of measuring important variables to improve clinical research and patient care, in view of thresholds of mechanical challenge that can be used to predict outcome and guide treatment.
Financial support was provided by grants from the National Institutes of Health (AR46601, AR48939, and AR55533), the Orthopaedic Research and Education Foundation, the Orthopaedic Trauma Association, and the Arthritis Foundation.