Subchondral Density Research Articles

A pilot study of four-year longitudinal bone changes following anterior cruciate ligament reconstructive surgery using DXA and HR-pQCT

Anterior cruciate ligament reconstructive (ACLR) surgery is an effective and prevalent treatment to restore function following ACL injury. The incidence of post-traumatic osteoarthritis (PTOA) long-term is still a major problem. Longitudinal studies beyond the first year are lacking and no studies have reported high-resolution in vivo bone changes. (1) Determine longitudinal bone changes following surgery in the proximal tibia and distal femur in ACLR and contralateral knees, and (2) determine bone changes at the ACLR tunnel and surrounding region in the injured knee. Participants with acute ACL-injuries (n = 5) had DXA and HR-pQCT imaging at four timepoints: pre-surgery (baseline) and at 2-, 6- and 48-months in the injured and contralateral knees. Areal BMD in peri-articular and epiphyseal regions was evaluated using DXA. Microstructural and vBMD changes of the tibial plateau and femoral condyles and surrounding the ACLR tunnel were determined. Large initial reductions in subchondral density and microstructure within the first year were followed by a period of recovery. Although varying among participants, reduced values in the injured knee remained at 48 months. Substantial adaptations to the surrounding ACLR tunnel bone occurred resulting in a cortex-like thickening and reduced wall diameters at 48 months. Large initial reductions following ACLR surgery followed a similar pattern seen in the first year after injury, indicating the trauma of surgery may reset bone recovery. These longitudinal results are consistent with cross-sectional studies indicating irreversible bone changes following ACLR that may play a role in PTOA initiation and progression. This pilot data suggests that imaging biomarkers are persistent at 48 months, possibly reflecting early PTOA changes.

Subchondral Bone Microarchitectural and Mineral Properties and Expression of Key Degradative Proteinases by Chondrocytes in Human Hip Osteoarthritis.

Background: The purpose of this study was to investigate the relationship between the expression of key degradative enzymes by chondrocytes and the microarchitectural and mineral properties of subchondral bone across different stages of cartilage degradation in human hip osteoarthritis (OA). Methods: Osteochondral samples at different stages of cartilage degradation were collected from 16 femoral heads with OA. Osteochondral samples with normal cartilage were collected from seven femoral heads with osteoporosis. Microcomputed tomography was used for the investigation of subchondral bone microarchitecture and mineral densities. Immunohistochemistry was used to study the expression and distribution of MMP13 and ADAMTS4 in cartilage. Results: The microarchitecture and mineral properties of the subchondral plate and trabecular bone in OA varied with the severity of the degradation of the overlying cartilage. Chondrocytes expressing MMP13 and ADAMTS4 are mainly located in the upper zone(s) of cartilage regardless of the histopathological grades. The zonal expression of these enzymes in OA (i.e., the percentage of positive cells in the superficial, middle, and deep zones), rather than their overall expression (the percentage of positive cells in the full thickness of the cartilage), exhibited significant variation in relation to the severity of cartilage degradation. The associations between the subchondral bone properties and zonal and overall expression of these enzymes in the cartilage were generally weak or nonsignificant. Conclusions: Phenotypic changes in chondrocytes and remodelling of subchondral bone proceed at different rates throughout the process of cartilage degradation. Biological influences are more important for cartilage degradation at early stages, while biomechanical damage to the compromised tissue may outrun the phenotypic change of chondrocytes and is critical in the advanced stages.

Subchondral and Trabecular Bone Respond Differently to Exercise in Juvenile Sheep

Evaluating the responses of subchondral and underlying trabecular bone properties to exercise interventions have implications for understanding normal developmental processes as well as the etiology of diseases affecting these tissues. We hypothesized that exercise would result in (1) increased subchondral and trabecular thickness and density, and increased bone volume fraction, (2) the distribution of increased bone properties would reflect experimentally induced postural differences, and (3) shallower trabecular bone would show a greater response to exercise than deeper trabecular bone. To evaluate these hypotheses we randomly assigned thirty juvenile sheep to flat exercise (n=11), inclined exercise (15% grade; n=11) which used more flexed knee postures, and non‐exercised control (n=8) groups. Exercise was conducted on motorized treadmills twice daily for 20 min at 1.12m/s for 60 days. Distal femora were micro‐CT‐scanned at 50μm resolution. Trabecular thickness, apparent density, bone volume fraction, anisotropy, as well as subchondral thickness and apparent density were measured using custom algorithms in AMIRA, ImageJ and MATLAB. Subchondral thickness showed significant and predictable response to exercise and with approximately 20% thicker bone in both exercise groups compared to controls, and postural changes with increased thickness toward the posterior side of the condyle in the incline group. Subchondral density did not increase substantially with exercise. In contrast trabecular density was increased slightly with exercise (~5%) but the response did not correspond to incline‐exercised and control groups. These results suggest a greater responsiveness to exercise in subchondral than trabecular bone, and imply that increases in stiffness in these tissues may be achieved through modulation of different tissue properties.Support or Funding InformationNational Science Foundation 1638756

Subchondral Density and Articular Cartilage Thickness in the Fetlock Joints of Thoroughbred Racehorses

Subchondral Density and Articular Cartilage Thickness in the Fetlock Joints of Thoroughbred Racehorses

Density of bones forming hip and knee joints in patients aged over 60 years with coxarthrosis and gonarthrosis according to multislice computed tomography

The purpose of the study was to review quantitative and qualitative radiological manifestations of coxarthrosis and gonarthritis in patients aged over 60 years. Radiography, computed tomography and multislice computed tomography were performed for 48 patients aged 60-70 years with deforming arthritis of the hip and knee joints. Idiopathic and dysplastic cases prevailed. The results of the study showed that roentgen anatomical changes in the hip and knee joints were accompanied by changes in the density of femoral head, typical restructuring of the femoral and tibial condyles, patella that could earlier be detected with computed tomography. A decrease in the total density of the femoral and tibial condyles and a relative increase in subchondral density with varus of the knee in particular were shown to be a specific feature of quantitative changes in the density of bone tissue in patients with deforming arthritis.

In vivo precision of a depth-specific topographic mapping technique in the CT analysis of osteoarthritic and normal proximal tibial subchondral bone density

To test the in vivo precision of a depth-specific topographic mapping technique (CT-TOMASD, computed tomography topographic mapping of subchondral density) for the 3D assessment of subchondral cortical and trabecular bone density in normal and osteoarthritic (OA) human tibiae. Fourteen participants were recruited (3 men:11 women; mean age: 49.9, SD: 11.9 years) and categorized as normal (n = 7) or OA (n = 7). Each participant was scanned using clinical quantitative CT (QCT) three times over 2 days. We assessed average subchondral bone mineral density (BMD) across three layers (0-2.5 mm, 2.5-5 mm and 5-10 mm) measured in relation to depth from the subchondral surface. Regional analyses included: medial plateau BMD; lateral plateau BMD; anterior/central/posterior compartment BMD; medial:lateral (M:L) BMD ratio; and average BMD of a 10-mm diameter "focal spot," which searched each medial and lateral plateau for the highest focal densities present within each plateau. Precision was assessed using root mean square coefficients of variation (CV%(RMS)) and intraclass correlation coefficients (ICC). Average CV%(RMS) precision errors for BMD measures were 2.3%, reaching a maximum CV%(RMS) of 3.9%. ICC showed high repeatability above 0.98. CT-TOMASD offered precise 3D measures of subchondral BMD. This method has the potential to identify and quantify changes in subchondral BMD associated with OA in vivo.

Subchondral Bone Apparent Density and Locomotor Behavior in Extant Primates and Subfossil Lemurs Hadropithecus and Pachylemur

We analyze patterns of subchondral bone apparent density in the distal femur of extant primates to reconstruct differences in knee posture, discriminate among extant species with different locomotor preferences, and investigate the knee postures used by subfossil lemur species Hadropithecus stenognathus and Pachylemur insignis. We obtained computed tomographic scans for 164 femora belonging to 39 primate species. We grouped species by locomotor preference into knuckle-walking, arboreal quadruped, terrestrial quadruped, quadrupedal leaper, suspensory and vertical clinging, and leaping categories. We reconstructed knee posture using an experimentally validated procedure of determining the anterior extent of the region of maximal subchondral bone apparent density on a median slice through the medial femoral condyle. We compared subchondral apparent density magnitudes between subfossil and extant specimens to ensure that fossils did not display substantial mineralization or degradation. Subfossil and extant specimens were found to have similar magnitudes of subchondral apparent density, thereby permitting comparisons of the density patterns. We observed significant differences in the position of maximum subchondral apparent density between leaping and nonleaping extant primates, with leaping primates appearing to use much more flexed knee postures than nonleaping species. The anterior placement of the regions of maximum subchondral bone apparent density in the subfossil specimens of Hadropithecus and Pachylemur suggests that both species differed from leaping primates and included in their broad range of knee postures rather extended postures. For Hadropithecus, this result is consistent with other evidence for terrestrial locomotion. Pachylemur, reconstructed on the basis of other evidence as a committed arboreal quadruped, likely employed extended knee postures in other activities such as hindlimb suspension, in addition to occasional terrestrial locomotion.

A comparison of conventional maximum intensity projection with a new depth-specific topographic mapping technique in the CT analysis of proximal tibial subchondral bone density

The objective was to identify subchondral bone density differences between normal and osteoarthritic (OA) proximal tibiae using computed tomography osteoabsorptiometry (CT-OAM) and computed tomography topographic mapping of subchondral density (CT-TOMASD). Sixteen intact cadaver knees from ten donors (8 male:2 female; mean age:77.8, SD:7.4 years) were categorized as normal (n = 10) or OA (n = 6) based upon CT reconstructions. CT-OAM assessed maximum subchondral bone mineral density (BMD). CT-TOMASD assessed average subchondral BMD across three layers (0-2.5, 2.5-5 and 5-10 mm) measured in relation to depth from the subchondral surface. Regional analyses of CT-OAM and CT-TOMASD included: medial BMD, lateral BMD, and average BMD of a 10-mm diameter area that searched each medial and lateral plateau for the highest "focal" density present within each knee. Compared with normal knees, both CT-OAM and CT-TOMASD demonstrated an average of 17% greater whole medial compartment density in OA knees (p < 0.016). CT-OAM did not distinguish focal density differences between OA and normal knees (p > 0.05). CT-TOMASD focal region analyses revealed an average of 24% greater density in the 0- to 2.5-mm layer (p = 0.003) and 36% greater density in the 2.5- to 5-mm layer (p = 0.034) in OA knees. Both CT-OAM and TOMASD identified higher medial compartment density in OA tibiae compared with normal tibiae. In addition, CT-TOMASD indicated greater focal density differences between normal and OA knees with increased depth from the subchondral surface. Depth-specific density analyses may help identify and quantify small changes in subchondral BMD associated with OA disease onset and progression.

Morphological adaptation of the tarso-metatarsal joints onto load transmission in the foot

Undefined middle foot pain among older individuals is an increasingly problem. Thereby the tarso-metatarsal joints (Lisfranc's joint, TMT joints) are particularly important for foot structure. The aim of this study was to obtain into force transmission in TMT joints by analysing degenerative morphological changes (DMC) of the articular cartilage and subchondral density patterns. We used seven formalin-fixed foot preparations (average age 80.8) from the undergraduate dissection course. Applying the Collins schema, localisation of cartilaginous surface DMC was examined. For showing subchondral density patterns, the well-established method of CT-osteoabsorptiometry (CT-OAM) was used. The cartilaginous surface DMCs were frequently located at the medial edges. Maximum density areas were found at the medial and dorsal edges. There was a clear correlation of topical cartilage degeneration and subchondral density. In healthy feet, force transmission in the TMT joints appears to occur via the dorsal and medial edges. Considering the correlation of topical cartilaginous surface DMC and subchondral density patterns, clinical application of CT-OAM seems to be recommendable in the case of undefined middle foot pain.

Computed tomography topographic mapping of subchondral density (CT-TOMASD) in osteoarthritic and normal knees: methodological development and preliminary findings

To develop a precise imaging tool which measures three-dimensional (3D) subchondral bone mineral density (BMD), and investigate its ability to distinguish subchondral bone properties in osteoarthritic and normal cadaveric tibiae. We developed a novel imaging tool [Computed tomography topographic mapping of subchondral density (CT-TOMASD)], which employs a surface projection image processing technique to map 3D subchondral BMD measured in relation to depth from the joint surface. Sixteen intact cadaver knees from 10 donors (8M:2F; age: 77.8+/-7.4) were scanned using quantitative computed tomography (QCT). Projections of average BMD to normalized depths of 2.5mm and 5.0mm were acquired, with regional analyses including: (1) medial and lateral BMD, (2) anterior/central/posterior compartmental BMD, (3) max BMD contained within a 10mm diameter 'core', and (4) medial:lateral BMD ratio. Precision was assessed using coefficients of variation (CV%). Osteoarthritis (OA) severity was assessed by examination of computed tomography (CT) and fluoroscopic radiographic images, and categorized using modified Kellgren-Lawrence (mKL) scoring. Precision errors for CT-TOMASD BMD measures were focused around 1.5%, reaching a maximum CV% of 3.5%. OA was identified in eight compartments of six knees. Substantial qualitative and quantitative differences were observed between the OA and normal knees, with the medial:lateral BMD ratio and peak core regional analyses demonstrating differences greater than 4.7 standard deviations (SDs) when compared with normals. Preliminary results revealed effect sizes ranging from 1.6 to 4.3 between OA and normal knees. CT-TOMASD offers precise 3D measures of subchondral BMD. Preliminary results demonstrate large qualitative and quantitative differences and large effect sizes between OA and normal knees. This method has the potential to identify and quantify changes in subchondral BMD associated with OA disease progression.

The distribution of cartilage degeneration of the human patella in relation to individual subchondral mineralization

According to Pauwels and Kummer the distribution of the subchondral mineralisation can be taken as a measure of the long-term mechanical loading in the joints. Based on a previous study concerning the density distribution of the subchondral bone of the human patella, these results were compared in the following paper with the macroscopically visible cartilage lesions. Degenerative changes on the lateral facet of the patella coincide with regions of high bone mineralisation, those on the "odd facet" on the other hand with areas of low subchondral density. The cartilage lesions on the lateral facet are seen as being due to high and constant mechanical stress, those on the "odd facet" to short infrequent stress-peaks.

Computed tomographic osteoabsorptiometry of the elbow joint in clinically normal dogs.

To evaluate subchondral bone density patterns in elbow joints of clinically normal dogs by use of computed tomographic (CT) osteoabsorptiometry. 20 cadaver forelimbs from 10 clinically normal dogs. Each elbow joint was imaged in parasagittal and transverse planes of 1.5-mm thickness. Slice data were converted to dipotassium phosphate equivalent density (PPED) values. Sagittal, parasagittal, and transverse medial coronoid process topographic maps were constructed. Defined zones were created for each of the 3 CT planes, and confluence and peak PPED values were determined. The lowest PPED value was 340 mg/ml (articular and subchondral confluence), and the highest was 1780 mg/ml (peak subchondral density). Detectable effects of joint laterality were not found in the confluence or peak PPED measurements or in the peak-to-confluence PPED ratio for all 3 CT planes. Significant differences were found among zones in all 3 planes for confluence and peak PPED measurements and between sagittal and transverse planes for peak-to-confluence PPED ratios. Subjectively, the pattern of density distribution among dogs was fairly consistent for the sagittal and parasagittal slices. Three specific patterns of density distribution were apparent on the transverse topographic maps of the medial coronoid process that corresponded to conformational differences. The use of CT osteoabsorptiometry provides a repeatable technique that can be used to noninvasively examine bone density and the effects of stress acting on joints in vivo. Variability in density values for any of the CT planes was not identified among clinically normal dogs.

Tension and bending, but not compression alone determine the functional adaptation of subchondral bone in incongruous joints.

In the present study, we tested the hypothesis that tension and bending, rather than compression alone, determine the functional adaptation of subchondral bone in incongruous joints. We investigated whether tensile stresses in the subchondral bone of the humero-ulnar articulation are affected by the direction of muscle and joint forces, and whether the tensile stresses are large enough to cause microstructural adaptation, specifically a preferential alignment of the trabeculae and the subchondral collagen fibres. Using a previously validated finite element model of the human humero-ulnar joint, we calculated the contact pressure, the principal compressive and tensile stresses, and the strain energy density in the subchondral bone for various flexion angles. A bicentric (ventro-dorsal) pressure distribution was found in the joint at 30 degrees to 120 degrees of flexion, with contact pressures of up to between 2.5 and 3 MPa in the ventral and dorsal aspects of the ulnar joint surface, but less than 0.5 MPa in the centre. The principal tensile stress in the subchondral bone of the trochlear notch quantitatively exceeded the principal compressive stress at low flexion angles (maximum 8.2 MPa), and the distribution of subchondral strain energy density differed substantially from that of the contact stress (r=-0.72 at 30 degrees and r=+0.58 at 90 degrees of flexion). No important tensile stress was computed in the trochlea humeri. On contact radiography, we found sagittally orientated subarticular trabeculae in the notch, running tangential to the surface. Furthermore, we observed sagittally orientated split lines in the subchondral bone of the notch of 20 cadaver joints, suggesting a ventro-dorsal orientation of the collagen fibres. The trochlea humeri, on the other hand, did not show a preferential direction of the subchondral split lines, these findings confirming the predictions of tensile stresses in the model. We conclude that, due to the important contribution of tension to subchondral bone stress, the distribution of subchondral density cannot be directly employed for assessing the long term distribution of joint pressure at the cartilage surface. The magnitude of the tensional stress varies considerably with the direction of the muscle and joint forces, and it appears large enough to cause functional adaptation of the subchondral bone on a microstructural level.

The subchondral bone plate.

Pauwels (1965) and subsequent workers in the same field have shown that the distribution of the subchondral density within a joint surface can serve as a parametric measurement which reflects the main stress acting on a joint. Our own investigations on anatomical specimens have demonstrated that this subchondral mineralization does indeed show regular distribution patterns from which conclusions about the mechanical situation within an individual joint may be drawn. Since radiographical densitometry and histological methods are only available for determining the adaptive reaction of the bone to the particular mechanical situation in a joint after death, the information obtained applies only to an end situation and tells us nothing about the development of the changes with time. Furthermore, investigations carried out on human specimens by radiographical densitometry mostly apply to samples of a particular age, since such specimens can be acquired only from departments of pathology, forensic medicine or anatomy. The functional reactions of the bone tissue to repeated long-term changes in the loading--lengthy immobilization and subsequent remobilization, for instance, or heavy loading over a considerable period of time--cannot be followed by any ordinary method in experimental animals, since the death of the animal is a prerequisite for the precise quantitative examination of the bone tissue. This applies also to attempts to follow the process by means of animal experiments. CT OAM has been developed as a method which, based on CT, can provide a surface representation of the 3-D density distribution in the joints of living subjects. Comparative studies were carried out to establish and confirm the validity of the procedure. These have shown (1) that the results obtained from anatomical specimens are identical with those obtained in the living; (2) that secondary CT sections are suitable for evaluation and that the spectrum of joint surfaces examined can be extended to include the whole joint (if this were not so, effects caused by the apparatus--particularly the partial-volume effect--would render the procedure impossible); and finally (3) that the distribution of the Hounsfield density within the subchondral bone represents the distribution of the mineralization. The mineralization patterns found by us in different joints of normal subjects have shown that these patterns can be brought into line with current models of joint mechanics. The radiocarpal joint, for instance, has revealed the various types of loading occurring within physiological limits. Information has also been obtained about the age-related changes taking place in the hip, wrist and ankle joints. The increase of the total mineralization in gymnasts can be related to the qualitative and quantitative adaptation to an increased peak loading, and reduced mineralization to a lengthy reduction in use during, for instance, postoperative immobilization. In groups of patients with various diseases of mechanical origin (shoulder instability, malalignment of the main axis, defective repositioning of healed fractures, rupture of the rotator cuff, meniscectomy or rupture of the anterior cruciate ligament), a pattern of mineralization is found which is different from the normal picture. These findings reflect the abnormal mechanical situation. The mineralization pattern of the femoropatellar joint has revealed the differing etiologies of medial and lateral cartilage damage and the examination of patients with lunatomalacia has made it possible to recognize a genetic disposition. The postoperative comparison of the mineralization patterns of patients with genu varum who have undergone a correction osteotomy and the results of animal experiments on various procedures for reconstructing the anterior cruciate ligament or a primary replacement of the meniscus, have produced results which make it possible to judge the success or failure of the operation. (ABSTRACT TRUNCATED)

Mechanobiological adaptation of subchondral bone as a function of joint incongruity and loading

Computed tomography (CT) has been employed to determine non-invasively the distribution of subchondral bone density in joints and to evaluate their dominant loading pattern. The objective of this study was to investigate the relationship between subchondral bone adaptation, joint incongruity and loading, in order to determine to what extent the loading conditions and/or geometric configuration can be inferred from the distribution of subchondral density. Finite element models of joints with various degrees of incongruity were designed and a current remodeling theory implemented using the node-based approach. Appropriate combinations of joint incongruity and loading yielded subchondral bone density patterns consistent with experimental findings, specifically a bicentric distribution in the humero-ulnar joint and a monocentric distribution in the humero-radial joint. However, other combinations of incongruity and loading produced similar subchondral density patterns. Both the geometric joint configuration and the loading conditions influence the distribution of subchondral density in such a way that one of these factors must be known a priori to estimate the other. Since subchondral density can be assessed by CT and joint geometry by magnetic resonance imaging, the dominant loading pattern of joints may be potentially derived in the living using these non-invasive imaging methods.

Mechanical implications of humero-ulnar incongruity — finite element analysis and experiment

Previous studies show that the humero-ulnar joint is physiologically incongruous [Eckstein et al. (1995a) Anat. Rec. 243, 318–326] and exhibits a bicentric (ventro-dorsal) distribution of subchondral mineralization [Eckstein et al. (1995b) J. Orthop. Res. 13, 286-278]. We therefore asked: (1) Does humero-ulnar incongruity bring about a bicentric distribution of contact pressure? (2) Do tensile stresses occur in the subchondral bone of the trochlear notch that are in the same order of magnitude as the compressive stresses? (3) Do ventral and dorsal maxima of subchondral bone density correlate with a bicentric distribution of strain energy density? To that end, a two-dimensional finite element model was designed. The shape and material properties of the bones were based on CT and the boundary conditions selected to agree with resisted elbow extension at 90° of flexion. The incongruity and contact areas were determined experimentally from casts, and the pressure distribution with Fuji Prescale film. In the model and the experiment contact stresses above 2 MPa were recorded in the ventral and dorsal parts of the joint, and values below 0.5 MPa in the depth of the notch. In the model, tensile stresses of 2.9 MPa were observed in the subchondral bone of the ulna, but not in the humerus. The subchondral strain energy density yielded a bicentric pattern in a model with homogeneous subchondral bone properties. It is shown that humero-ulnar incongruity brings about a bicentric distribution of contact pressure, a tensile stress in the notch that is in the same order of magnitude as the compressive stress, and a distribution of strain energy density that correlates with subchondral density patterns.

Anatomic characteristics of the pelvic girdle

The most recent methods of investigation (CT osteoabsorptiometry) were applied in an attempt to reconstruct from their morphological structure the way in which connections between the bones of the pelvic girdle undergo stress. Thus, both sacroiliac articulation and symphysis pubis show characteristic distribution of the subchondral bone density and layout of the tensile collagen fibrous material as expression of a strongly varying qualitative pattern of stress during walking. In the region of sacroiliac articulation are the highest subchondral densities, both at the cranial and caudal edges, whereas the central part of the two auricular surfaces is less heavily mineralized. This distribution matches the thickness of the hyaline cartilage of the joint. There are striking sexual differences in the distribution of the subchondral bone density at the bordering surfaces of the symphysis pubis. During walking, all components of sacroiliac articulation and the symphysis pubis are apparently subjected to sudden changes in stress. Independent of this, the os sacrum is constantly exposed to torque on account of the weight of the upper body, and this is balanced out by the sacrospinal and sacrotuberal ligaments.

Computed tomography-osteoaboorptiometry: a method of assessing the mechanical condition of the major joints in a living subject

A method of making a visual display of subchondral mineralization in the major synovial joints is described. Unlike existing procedures, it can be used on the living subject. A modified application of computed tomography-densitometry, computed tomography-osteoabsorptiometry makes it possible to explore the mechanical adaptability to the prevailing mechanical force. This claim is based upon the comparison of information obtained from 20 anatomical specimens with CT-osteoabsorptiometry and x-ray densitometry of sections; both methods yielding virtually identical results. The distribution of the subchondral density was then expressed as a map of the articular surface with the aid of an image analyser. This method can make a useful contribution to basic clinical research, as well as providing a diagnostic technique which can also be used for observing progress after a corrective osteotomy or any other procedure causing a change in mechanical function. Examples of its use on living patients are given.

The third carpal bone

equal prevalence of distal radial carpal (DRC) and third carpal (C3) injury but in Standardbreds, C3 is more commonly damaged. The medial aspect of the intercarpal joint is highly susceptible to overload because of the greater load on the medial aspect of the joint and the hinge-type closing of the joint. The typical distalradial carpal (DRC) chip fracture occurs in the middle of its articular rim just lateral or adjacent to the attachment of a small ligament that courses distally and medially to merge with the capsular attachments to the face of the second carpal bone. The typical chip is therefore best visualized on the DLPMO and flexed lateral views. The flexed view is important to assess since loss of normal subchondral density extending palmarly away from the fracture is a sign of extensive articular damage and a poor prognosis. Thebest DRC chips are triangular in outline and relatively narrow along their articular margin. A second very common site of DRC injury is the dorsal lateral comer of the bone. This is the most common site for an osteophyte to develop (and subsequently fracture) in the horse. Such lesions can be difficult to diagnose radigraphically and are best seen on a well-positioned, well-exposed skyline or a wellpenetrated DMPLO projection. They are rarely found as a solitary lesion. Arthroscopic removal is the preferred management for most carpal fractures although it certainly should be acknowledged that many small carpal chips can be managed medically. If surgery is an option, in terms of the horse's value, repeated injections should be avoided and, in particular, repetitive corticosteroids should be avoided. Early intervention affords a better prognosis so a decision concerning surgery should be made shortly after diagnosis. Following ar throscopic surgery for atypical chip fracture, horses are rested for about two weeks with hand grazing only allowed. They then begin a program of handwalking that begins with five minutes/day and increases weekly by five minute increments until they are walking 30-40 minutes/day. Swimming is also a rehabilitative option that can begin any time after two weeks. Passive flexion is advised for the first month following surgery. Sodium hyaluronate is usually injected five to seven days following surgery and repeated when the horse returns to training if an effusion persists. A horse with a typical distal lateral radius chip fracture will start jogging about six weeks postoperatively, whereas most serious intercarpal injuries receive two to three times as long a convalescence. THE THIRD CARPAL BONE