Thomas E. Baer

THE ROLE OF THE INTEROSSEOUS TALOCALCANEAL LIGAMENT IN SUBTALAR JOINT STABILITY

Background: Injury of the interosseous talocalcaneal ligament (ITCL) has been recognized as a cause of subtalar instability, though lack of an accepted clinical test has limited the ability of clinicians to reliably make the diagnosis. Clinical effects of ITCL failure remain unclear because of insufficient understanding of the role of the ligament. Methods: Load-displacement characteristics of the subtalar joint were studied in six cadaver specimens using an axial distraction test and a transverse multi-direction drawer test. In all tests, cyclic loading (+/−60 N) was applied, and load-displacement responses were collected before and after sectioning of the ITCL. Two parameters were used to analyze the data: neutral-zone laxity as a measure of joint play, and flexibility as a measure of resistance to applied force. Results: In the axial distraction test, sectioning increased both neutral-zone laxity and flexibility (p = .01 and .02, respectively). In the transverse test, sectioning caused increase of both neutral-zone laxity and flexibility (p <.001, for each). Neutral-zone laxity increased most greatly along an axis defined roughly by the posterior aspect of the fibula and the central region of the medial malleolus. Flexibility increased most in the medial direction (p <.05, for each). Conclusions: Results confirmed the role of the ITCL in maintaining apposition of the subtalar joint, as well as suggested its role in stabilizing the subtalar joint against drawer forces applied to the calcaneus from lateral to medial. The dominant direction of increased neutral-zone laxity described above suggests the optimal direction for detecting subtalar instability involved with ITCL injury. Clinical Relevance: ITCL failure may result in subtalar instability and should be examined with a drawer force along the preferential axis roughly from the posterior aspect of the fibula to the central region of the medial malleolus. Further clinical evaluation is required to determine whether ITCL failure is reliably detectable.

Hip Joint Contact Force in the Emu (Dromaius novaehollandiae) during Normal Level Walking

The emu is a large, (bipedal) flightless bird that potentially can be used to study various orthopaedic disorders in which load protection of the experimental limb is a limitation of quadrupedal models. An anatomy-based analysis of normal emu walking gait was undertaken to determine hip contact forces for comparison with human data. Kinematic and kinetic data captured for two laboratory-habituated emus were used to drive the model. Muscle attachment data were obtained by dissection, and bony geometries were obtained by CT scan. Inverse dynamics calculations at all major lower-limb joints were used in conjunction with optimization of muscle forces to determine hip contact forces. Like human walking gait, emu ground reaction forces showed a bimodal distribution over the course of the stance phase. Two-bird averaged maximum hip contact force was approximately 5.5 times body weight, directed nominally axially along the femur. This value is only modestly larger than optimization-based hip contact forces reported in literature for humans. The interspecies similarity in hip contact forces makes the emu a biomechanically attractive animal in which to model loading-dependent human orthopaedic hip disorders.

Incongruity-dependent changes of contact stress rates in human cadaveric ankles.

Summary: Cartilage biosynthetic transduction and injury characteristics have been shown to be particularly sensitive to changes in contact stress rates. This study investigated incongruity-associated changes in contact stress rates that resulted from an articular surface stepoff of the distal tibia in human cadaveric ankles. Ten human cadaveric ankles were subjected to quasi-physiologic stance-phase motion and loading and instantaneous contact stresses were captured at 132 Hz over the entire articular surface using a custom-fabricated stress transducer. An osteoarticular fragment consisting of the anterolateral 25% of the distal tibia was osteotomized. Testing was repeated after displacing the fragment proximally between 0.0 mm to 4.0 mm in 1.0 mm increments. Transient contact stress measurements were used to calculate contact stress rates. Compared to intact ankles, the anatomic configuration had modest increases in global and peak postitive and negative contact stress rates throughout the motion cycle. In contrast, stepoff specimens had significant increases in global and complete motion cycle peak positive and negative contact stress rates, as high as 3.1X intact values in specimens with a 4.0 mm stepoff. Contour plots of contact stress rates also demonstrated an instability event during motion. An anterolateral stepoff of the distal tibia caused significant changes in positive and negative contact stress rates in cadaveric ankles. Incongruity-associated changes in contact stress rates and incongruity-associated instability events may be important pathomechanical determinants of post-traumatic arthritis.

Effects of episodic subluxation events on third body ingress and embedment in the THA bearing surface.

In total joint arthroplasty, third body particle access to the articulating surfaces results in accelerated wear. Hip joint subluxation is an under-recognized means by which third body particles could potentially enter the otherwise closely conforming articular bearing space. The present study was designed to test the hypothesis that, other factors being equal, even occasional events of femoral head subluxation greatly increase the number of third body particles that enter the bearing space and become embedded in the acetabular liner, as compared to level-walking cycles alone. Ten metal-on-polyethylene hip joint head-liner pairs were tested in a multi-axis joint motion simulator, with CoCrMo third body particles added to the synovial fluid analog. All component pairs were tested for 2h of level walking; half were also subjected to 20 intermittent subluxation events. The number and location of embedded particles on the acetabular liners were then determined. Subluxation dramatically increased the number of third body particles embedded in the acetabular liners, and it considerably increased the amount of scratch damage on the femoral heads. Since both third body particles and subluxation frequently occur in contemporary total hip arthroplasty, their potent synergy needs to be factored prominently into strategies to minimize wear.

A clinically realistic large animal model of intra-articular fracture that progresses to post-traumatic osteoarthritis

OBJECTIVE Translation of promising treatments for post-traumatic osteoarthritis (PTOA) to patients with intra-articular fracture (IAF) has been limited by the lack of a realistic large animal model. To address this issue we developed a large animal model of IAF in the distal tibia of Yucatan minipigs and documented the natural progression of this injury. DESIGN Twenty-two fractures were treated using open reduction and internal fixation with either an anatomic reduction or an intentional 2-mm step-off. Pre-operatively, and 3 days, 1, 2, 4, 8, and 12 weeks post-operatively, animals were sedated for synovial fluid draws and radiographs. Limb loading was monitored at the same time points using a Tekscan Walkway. Animals were sacrificed at 12 weeks and the limbs were harvested for histological evaluation. RESULTS All animals achieved bony union by 12 weeks, facilitating nearly complete recovery of the initial 60% decrease in limb loading. TNFα, IL1β, IL6, and IL8 concentrations in the fractured limbs were elevated (P < 0.05) at specific times during the 2 weeks after fracture. Histological cartilage degeneration was more severe in the step-off group (0.0001 < P < 0.27 compared to normal) than in the anatomic reconstruction group (0.27 < P < 0.99 compared to normal). CONCLUSIONS This model replicated key features of a human IAF, including surgical stabilization, inflammatory responses, and progression to osteoarthritic cartilage degeneration, thereby providing a potentially useful model for translating promising treatment options to clinical practice.

Volar/dorsal compressive mechanical behavior of the transverse carpal ligament.

Mechanical insult to the median nerve caused by contact with the digital flexor tendons and/or carpal tunnel boundaries may contribute to the development of carpal tunnel syndrome. Since the transverse carpal ligament (TCL) comprises the volar boundary of the carpal tunnel, its mechanics in part govern the potential insult to the median nerve. Using unconfined compression testing in combination with a finite element-based optimization process, nominal stiffness measurements and first-order Ogden hyperelastic material coefficients (μ and α ) were determined to describe the volar/dorsal compressive behavior of the TCL. Five different locations on the TCL were tested, three of which were deep to the origins of the thenar and hypothenar muscles. The average (± standard deviation) low-strain and high-strain TCL stiffness values in compression sites outside the muscle attachment region were 3.6 N/mm (±2.7) and 28.0 N/mm (±20.2), respectively. The average stiffness values at compression sites with muscle attachments were notably lower, with low-strain and high-strain stiffness values of 1.2 N/mm (±0.5) and 9.7 N/mm (±4.8), respectively. The average Ogden coefficients for the muscle attachment region were 51.6 kPa (±16.5) for μ and 16.5 (±2.0) for α, while coefficients for the non-muscle attachment region were 117.8 kPa (±86.8) for μ and 17.2 (±1.6) for α. These TCL compressive mechanical properties can help inprove computational models, which can be used to provide insight into the mechanisms of median nerve injury leading to the onset of carpal tunnel syndrome symptoms.

Organ-Level Histological and Biomechanical Responses from Localized Osteoarticular Injury in the Rabbit Knee

The processes of whole‐joint osteoarthritis development following localized joint injuries are not well understood. To demonstrate this local‐to‐global linkage, we hypothesized that a localized osteoarticular injury in the rabbit knee would not only cause biomechanical and histological abnormalities in the involved compartment but also concurrent histological changes in the noninvolved compartment. Twenty rabbits had an acute osteoarticular injury that involved localized joint incongruity (a 2‐mm osteochondral defect created in the weight‐bearing area of the medial femoral condyle), while another 20 received control sham surgery. At the time of euthanasia at 8 or 16 weeks post‐surgery, the experimental knees were subjected to sagittal‐plane laxity measurement, followed by cartilage histo‐morphological evaluation using the Mankin score. The immediate effects of defect creation on joint stability and contact mechanics were explored in concomitant rabbit cadaver experimentation. The injured animals had cartilage histological scores significantly higher than in the sham surgery group (p < 0.01) on the medial femoral, medial tibial, and lateral femoral surfaces (predominantly on the medial surfaces), accompanied by slight (mean 20%) increase of sagittal‐plane laxity. Immediate injury‐associated alterations in the medial compartment contact mechanics were also demonstrated. Localized osteoarticular injury in this survival animal model resulted in global joint histological changes.

The apparent critical isotherm for cryoinsult-induced osteonecrotic lesions in emu femoral heads.

Cryoinsult-induced osteonecrosis (ON) in the emu femoral head provides a unique opportunity to systematically explore the pathogenesis of ON in an animal model that progresses to human-like femoral head collapse. Among the various characteristics of cryoinsult, the maximally cold temperature attained is one plausible determinant of tissue necrosis. To identify the critical isotherm required to induce development of ON in the cancellous bone of the emu femoral head, a thermal finite element (FE) model of intraoperative cryoinsults was developed. Thermal material property values of emu cancellous bone were estimated from FE simulations of cryoinsult to emu cadaver femora, by varying model properties until the FE-generated temperatures matched corresponding thermocouple measurements. The resulting FE model, with emu bone-specific thermal properties augmented to include blood flow effects, was then used to study intraoperatively performed in vivo cryoinsults. Comparisons of minimum temperatures attained at FE nodes corresponding to the three-dimensional histologically apparent boundary of the region of ON were made for six experimental cryoinsults. Series-wide, a critical isotherm of 3.5 degrees C best corresponded to the boundary of the osteonecrotic lesions.

Sliding Direction Dependence of Polyethylene Wear for Metal Counterface Traverse of Severe Scratches

Third-body effects appear to be responsible for an appreciable portion of the wear rate variability within cohorts of patients with metal-on-polyethylene joint replacements. The parameters dominating the rate of polyethylene debris liberation by counterface scratches are not fully understood, but one seemingly contributory factor is the scratchs orientation relative to the direction of instantaneous local surface sliding. To study this influence, arrays of 550 straight parallel scratches each representative of the severe end of the clinical range were diamond stylus-ruled onto the surface of polished stainless steel plates. These ruled plates were then worn reciprocally against polyethylene pins (both conventional and highly cross-linked) at traverse angles varied parametrically relative to the scratch direction. Wear was measured gravimetrically, and particulate debris was harvested and morphologically characterized. Both of the polyethylene variants tested showed pronounced wear rate peaks at acute scratch traverse angles (15 deg for conventional and 5 deg for cross-linked), and had nominally comparable absolute wear rate magnitudes. The particulate debris from this very aggressive test regime primarily consisted of extremely large and elongated strands, often tens or even hundreds of microns in length. These data suggest that counterface damage regions with preferential scratch directionality can liberate large amounts of polyethylene debris, apparently by a slicing/shearing mechanism, at critical (acute) attack angles. However, the predominant manifestation of this wear volume was in the form of particles far beyond the most osteolytically potent size range.

Clinical tip: development of an ankle distraction device compatible with MRI and radiography.

Measurements of cartilage thickness in human ankles are important in evaluating the development of osteoarthritis, particularly following articular fractures. Unlike the knee, cartilage in the ankle is relatively thin (on the order of 1.5 mm), so it is difficult to image clearly using either MRI or CT techniques. Part of the problem is the lack of separation between the surfaces of the tibial and talar cartilage, making it essentially impossible to determine their respective thicknesses. Ankle distraction has been achieved in the operating room by applying tension to the foot using a windlass attached to the operating table, while the patient’s upper body is restrained by straps. Trauma units use devices like the Hare Traction splint, which applies force to the foot while bracing against the posterior ischium, to temporarily distract fractures of the femur. Neither of these systems is suitable for use with MRI, and significant modifications would be required to use them during CT scans. We have built a novel device that distracts the ankle (as well as the knee) using a specially modified brace above the knee to grip the distal surfaces of the thigh musculature where it narrows above the knee as anchor points (Figure 1) and a screw to apply tension to the foot strap. The distance between the tensioning mechanism and the brace is adjustable for patient size. The parts of the distractor adjacent to the ankle are radiolucent and the entire device is compatible with the MR and CT environments (Figure 2). In use, the device has been found to be minimally uncomfortable for the patient. Fig 1 Arrows indicate anchor points for the knee brace on the musculature of the distal thigh. Fig 2 The distractor in place on a subject leg To facilitate application of consistent tension to the ankle, a simple proving ring-based gauge made entirely of plastic was added to the system (Figure 3). It is placed in the load train between the tensioning screw and the ankle strap. As the force increases, the circular ring distorts and the rack attached to one side of the hoop moves relative to the pinion on the other. This action turns the attached calibrated scale, and the force is read against a mark on the rim of the hoop. Figure 3 The tension gauge is connected between the foot strap (left) and the tensioning screw (right). The gauge ensures that consistent loads are applied to the ankle and allows the technician to monitor any slippage that would cause the force to diminish. As a matter of patient safety, forces are not allowed to exceed 30 pounds (130N). Using the device, we have greatly improved the quality of our MRI scans. Distraction of the ankle joint introduces enough separation between the cartilage surfaces for accurate segmentation of the digitized image for finite element modeling, as well as for direct visual analysis (Figure 4). CT scans also benefit from use of the device (Figure 5). Figure 4 Coronal plane MRI scans showing distraction of the ankle mortise. Anatomical (left), and distracted (right). Figure 5 Sagittal plane CT scans showing distraction between the tibia and talus. Anatomical (left), and distracted (right).