The force and contact stress on the navicular bone during trot locomotion in sound horses and horses with navicular disease

Mechanical overload due to poor conformation or shoeing has been suggested to contribute to the development of navicular disease. While studies have determined the compressive force exerted on the navicular bone in normal horses, this has not been reported for horses with navicular disease. Also, the force has not been converted to stress by correction for contact area. In this study we developed a technique for the determination of the contact area between the deep digital flexor tendon and the navicular bone in vivo, and used a forceplate system to determine the force and stress on the bone at trot in 6 normal and eight diseased horses. The mean +/- s.d. peak force and peak stress were 5.62+/-1.45 N/kg and 2.74+/-0.76 MPa for the normal group and 6.97+/-1.50 N/kg and 3.07+/-0.55 MPa for the navicular disease group. The peak force and peak stress were similar for both groups but the force and stress in the horses with navicular disease were approximately double control group values early in the stance phase. This was due to a higher force in the deep digital flexor tendon, which was attributed to a contraction of the deep digital flexor muscle in early stance in an attempt to unload the heels.     

Comparison between magnetic resonance imaging and histological findings in the navicular bone of horses with foot pain

Reasons for performing study: There is limited knowledge about both histological features in early navicular disease and what histological features are represented by increased signal intensity in fat‐suppressed magnetic resonance (MR) images of the navicular bone. Objective: To characterise increased signal intensity in the spongiosa of the navicular bone in fat‐suppressed MR images and to compare this with histopathology; and to compare objective grading of all aspects of the navicular bone on MR images with histological findings. Methods: One or both front feet of 22 horses with foot pain and a median lameness duration of 3 months were examined using high‐field MR imaging (MRI) and histopathology. The dorsal, palmar, proximal and distal borders of the navicular bone and the spongiosa were assigned an MRI grade (0–3) and a histological grade and compared statistically. Results: Increased signal intensity in the spongiosa of the navicular bone was associated with a variety of abnormalities, including fat atrophy, with lipocytes showing loss of definition of cytoplasmic borders, a proliferation of capillaries within the altered marrow fat, perivascular or interstitial oedema, enlarged intertrabecular bone spaces, fibroplasia and thinned trabeculae showing loss of bone with irregularly spiculated edges of moth‐eaten appearance. There were significant associations among histological lesions of the fibrocartilage, calcified cartilage and subchondral bone. There were also significant associations between MRI grading of the spongiosa and both histological marrow fat grade and the combined maximum of the MRI grades for the fibrocartilage. Conclusions and potential relevance: Increased signal intensity in the spongiosa of the navicular bone in fat‐suppressed MR images may occur in association with lesions of the fibrocartilage with or without subchondral bone or may represent a separate disease entity, particularly if diffuse, reflecting a variety of alterations of trabecular bone and marrow fat architecture.     

The effect of bilateral palmar digital nerve analgesia on the compressive force experienced by the navicular bone in horses with navicular disease

Horses with navicular disease have an increased load on the navicular bone in early stance. This has been suggested to be a response to pain in the heel region. Seven horses with clinical, radiographic and scintigraphic signs of navicular disease underwent forceplate and kinematic analysis before and after desensitisation of the heel region with a bilateral palmar digital nerve block. The compressive force exerted on the navicular bone during stance, and stride kinematics, were determined in each state. After regional analgesia of the palmar digital nerves (PDNB) the compressive force on the navicular bone was lower throughout stance. The mean +/- s.d. peak force at the beginning of stance was 7.05+/-1.10 N/kg before, and 6.46+/-1.15 N/kg after PDNB (P = 0.01) and at the end of stance the mean peak values were 5.00+/-2.05 N/kg before, and 4.39+/-1.65 N/kg after PDNB (P = 0.05). We explained this finding as indicating that the horse responds to heel pain (including pain in the navicular region) by contracting the deep digital flexor muscle to unload the heels. This increases the compressive load on the navicular bone, which may cause remodelling and, in some horses, damage to the overlying flexor cartilage, which is then painful and identified as navicular disease. This mechanism identifies navicular disease as a possible end point for a variety of heel related conditions.     

The arterial supply of the navicular bone in adult horses with navicular disease

A macroscopical, arteriographical and histological study was made of the arterial supply of the navicular bones of horses with clinical and/or radiographic signs of navicular disease. Based on the clinical and radiological findings the navicular bones of 40 Warmblood horses are divided into four different groups. In the pattern of the arteriogram obvious changes are noticed: an imbalance and a distal to proximal shift between the distal and proximal arterial supply occur, indicating a reduction of the distal blood supply with a compensatory reaction of the proximal, medial and lateral supply. The changes in the pattern of the arteriogram are histologically defined by arterio(lo) sclerosis and newly formed arteries. The presence of radiologically visible nutrient foramina is associated with a changed pattern in the arteriogram, increased bone remodelling and fibrosis. Ischaemia and increased pressure (hypertension and/or increased intra-articular pressure) are considered to be responsible for changes in form and number of radiologically visible nutrient foramina.     

High-field magnetic resonance imaging investigation of distal border fragments of the navicular bone in horses with foot pain

Reasons for performing study: Distal border fragments of the navicular bone can be seen in lame and nonlame horses and their clinical significance remains open to debate. Objectives: To describe the magnetic resonance imaging (MRI) appearance of distal border fragments and the adjacent navicular bone. To investigate the relationship between fragments and other abnormalities of the navicular bone and the distal sesamoidean impar ligament (DSIL). Methods: Horses were included if pain causing forelimb lameness was localised to the foot and high‐field MR images were acquired. The size and location of distal border fragments were recorded. Abnormalities in the adjacent navicular bone were graded to obtain a fragment grade. A total navicular bone grade was assigned. The DSIL was also graded. A Chi‐squared test was used to test for associations between the presence of a fragment and specific lesions involving the distal border of the navicular bone, the total grade of the navicular bone, and the grade of the DSIL. Results: 427 horses were included and 111 fragments observed. There was a significant association between the presence of a fragment and the total navicular bone grade, osseous cyst‐like lesions, increased number and size of the synovial invaginations of the distal border, increased signal intensity on fat suppressed images and size of distal border entheseophytes. Conclusions: There is an association between distal border fragments and other pathological MRI abnormalities of the navicular bone. Potential relevance: Distal border fragments are part of navicular disease, but their contribution to pain and lameness remains to be clarified.     

Comparative biokinematic study of young and adult Andalusian horses at the trot

The purpose of this study was to determine basic kinematic parameters (linear, temporal and angular) in young and adult Andalusian horses (P.R.E.) at the trot, using a normal computer-aided videography system. The trotting gaits of 16 horses were analysed: seven young horses (3.7 +/- 0.2 years old, height at withers 167.1 +/- 4.1 cm) and nine adult stallions (12.3 +/- 2.9 years old, height at withers 162.9 +/- 3.6 cm) were recorded at least 6 times at the trot using a 25-Hz video-camera filming from the side. Video images were processed with a real-time digital system (SMVD). Speeds averaged 3.84 and 3.75 m/s for young and adult horses, respectively. Differences between age groups for speed and linear and temporal parameters of the stride were not significant. However, variations in angular parameters were detected: adults showed a greater ARM than younger horses for most forelimb joints. In the hind limb, hip, stifle, and, to a lesser degree, the tarsal joint, a smaller degree of extension during the stance phase was observed in adult horses.     

Temporal and linear kinematics in elite and riding horses at the trot

Standard videography was used to determine the temporal and linear variables of 32 Spanish Purebreds at trot. Horses were organized into three groups: Group A (nonelite), comprising 13 stallions without any specific training program, which were hand led; Group B1 (elite), formed by 19 horses considered to be well-gaited and dressage trained, and which were also hand led; and Group B2 (ridden elite), comprising the same horses as the latter group but this time ridden. The horses ran at working trot, according to the trainers. Each animal was recorded using a Hi8 video camera. For temporal measurements, a 1/50 s time code was added to video images. Recordings were analyzed to provide times (seconds) at landing, mid-stance position, lift off and at subsequent landing in each limb. These data allowed the stride to be divided into its component phases by analysis of the variables for individual limbs, diagonal pairs and overall stride. The linear variables (meters) analyzed were stride length and overtracking, obtained for each stride from digitized images and calculated on a spreadsheet. Data were then compared between Groups A-B1 and Groups B1–B2.

Stride duration (0.639±0.064 vs 0.672±0.037 s) and stride length (247.7±21.9 vs 238.5±18.1 cm) of horses from A and B1 groups were similar; Group B1 was characterized by limb support and overlap values close to those of Group A; however, limb swing (forelimbs: 0.391±0.032 s; hind limbs: 0.374±0.027 s), diagonal swing (0.365±0.027 s) and inter-overlap (0.131±0.035) were of greater duration. Additionally, the proportion of horses with advanced placement was greater in Group B1.

Group B2 displayed greater stride duration (0.718± 0.054 s) and lower stride length (217.3±26.1 cm) than B1 group. The analysis revealed mounted horses to be characterized by limb and diagonal swings similar to those of Group B1; however, limb support and overlap were significantly greater, and inter-overlap and suspension phase were shorter.     

Changes in digital venous pressures of horses moving at the walk and trot

Blood pressures from the catheterized lateral digital vein of the fore-limbs of 6 clinically normal horses were measured at rest, at the walk, and at the trot. Digital venous pressures were compared with the phases of the stride and weight-bearing forces, using electrogoniometry and a force platform. Rapid increases in digital venous pressures to maximal values were observed immediately before maximal forces during the support period of the stride. At the trot, increases in peak vertical forces were paralleled by increases in peak digital venous pressures. Seemingly, the hydrodynamics of the digital circulatory system help to dissipate the initial impact of hoof strike at the walk and the trot.     

Use of force plate analysis to assess the analgesic effects of etodolac in horses with navicular syndrome

OBJECTIVE: To evaluate the musculoskeletal analgesic effect of etodolac administered PO every 12 or 24 hours in chronically lame horses by use of force plate analysis. ANIMALS: 22 horses with navicular syndrome. PROCEDURE: Horses received etodolac (23 mg/kg, PO, q 12 h; n = 7), etodolac (23 mg/kg, PO, q 24 h; 8), or corn syrup (20 mL, PO, q 24 h; control treatment; 7) for 3 days. Combined forelimb peak vertical ground reaction force (PVF) was measured via force plate analysis before the first treatment (baseline) and at 6, 12, 24, and 36 hours after the last treatment. Differences in mean PVF (mPVF) between baseline and subsequent measurements were analyzed (repeated-measures ANOVA) and evaluated for treatment and time effects and treatment-time interaction. RESULTS: Once- or twice-daily administration of etodolac resulted in significant increases in mPVF from baseline at 6, 12, and 24 hours after the last treatment, compared with the control treatment. There were no significant differences in mPVF between the etodolac treatment groups at any time point. In both etodolac treatment groups, there was a significant increase in mPVF from baseline at 6, 12, and 24 hours, compared with that at 36 hours. Etodolac-associated adverse effects were not detected. CONCLUSIONS AND CLINICAL RELEVANCE: In horses with navicular syndrome, once-daily oral administration of 23 mg of etodolac/kg appears to provide effective analgesia for as long as 24 hours. Twice-daily administration of etodolac at this same dose does not appear to provide any additional analgesic efficacy or duration of effect.     

Lag screw fixation of sagittal navicular bone fractures in five horses

A sagittal fracture of the navicular bone in five horses was treated by means of lag screw. Intra-operative radiographic monitoring and a specially developed guide apparatus was necessary to implant the screw precisely along the transverse axis of the navicular bone. Internal fixation resulted in progressive radiographic narrowing of the fracture zone. In all five patients the fracture healed (bony union) without superfluous callus formation.     

Development of the navicular bone in foetal and young horses, including the arterial supply

A macroscopic, arteriographic and histological study of the development and the arterial anatomy of the navicular bone of 33 foetuses and 55 young horses is described. After 125 days of gestation the blood supply consists of two routes: one situated in the superficial layer of the fibrocartilage and the other similar to the blood supply of the navicular bone of the normal mature horse. After 270 days gestation, the blood vessels in the fibrocartilage gradually regressed and retracted until they have disappeared at six months after birth. At two months after birth the first macroscopic thinning of the fibrocartilage was noticed. From seven months to one year about 45 per cent of the navicular bones showed a slight thinning of the fibrocartilage. A positive correlation was found between radiographic abnormalities (ie enlargement of the nutrient foramina) and the frequency of thinning of the fibrocartilage. Radiographic abnormalities were first recognised 14 days after birth, whereas the arteriogram showed the first changes such as fewer or no arteries entering distally at the distal extremities at 10 weeks after birth. At four weeks after birth the first arterial wall changes were found, ie intimal thickening with or without splitting of the internal elastic membrane. From that age onward, the number of navicular bones with arterial wall changes gradually increased. Starting at five months after birth only 6 to 20 per cent of the arteries in the navicular bones without radiographic abnormalities showed arterial wall changes. However, the navicular bones with radiographic abnormalities showed arterial wall changes in 25 to 80 per cent of the arteries.     

Distal border fragments and shape of the navicular bone: radiological evaluation in lame horses and horses free from lameness

Reasons for performing study: The significance of distal border fragments of the navicular bone is not well understood. There are also no objective data about changes in thickness and proximal/distal extension of the palmar cortex of the navicular bone. Objectives: To describe the distribution of distal border fragments and their association with other radiological abnormalities of the navicular bone and describe the shape of the navicular bone in sound horses and horses with foot‐related lameness, including navicular pathology. Methods: Sound horses had radiographs acquired as part of a prepurchase examination. Lame horses had forelimb lameness abolished by palmar nerve blocks performed at the base of the proximal sesamoid bones. Diagnosis was assigned prospectively based on results of local analgesia and all imaging findings. The thickness of the palmar cortex of the navicular bone and size of proximal/distal extensions were measured objectively. Other radiological abnormalities were evaluated subjectively and each navicular bone graded. Results: Fifty‐five sound and 377 lame horses were included. All measurements were larger in lame compared with sound horses except the size of the distal extension of the palmar cortex. Fragments were observed in 3.6 and 8.7% of sound and lame horses respectively and in 24.1% of horses with a diagnosis of primary navicular pathology. There was an association between fragments and overall navicular bone grade, radiolucent areas at the angles of the distal border of the navicular bone and number and size of the synovial invaginations. Conclusions and potential relevance: The palmar cortex of the navicular bone was thicker in lame compared with sound horses. Distal border fragments were most frequent in horses with navicular pathology. Evaluation of changes in shape of the navicular bone may also be important for recognition of pathological abnormalities of the bone.     

Diagnosis and treatment of the navicular syndrome in horses

Navicular syndrome can be treated in a variety of ways. This is related to the fact that it has a variety of causes. Prognostically, most horses will improve with treatment. One can expect about 50 per cent of the horses to become useably sound for 1 year, no matter what treatment is used. The disease is progressive, and affected horses eventually will need to be retired because of lameness. The author's therapeutic approach is to utilize shoeing as the primary therapy. Shoeing is performed to correct structural problems and to ensure that shoeing is physiologically sound. Nonsteroidal anti-inflammatory drugs are not used unless radical changes have been made in the shoeing. In cases of confirmed distal interphalangeal joint synovitis, either sodium hyaluronate or polysulfated glycosaminoglycans will be used in conjunction with shoeing. In cases where decreased circulation is documented, isoxsuprine hydrochloride will be administered if shoeing alone has not improved the horse within 6 weeks. If therapy does not improve the horse within 6 to 12 weeks, palmar digital neurectomy is recommended.     

High‐field (3 Tesla) MRI of the navicular apparatus of sound horses shows good agreement to histopathology

Magnetic resonance imaging and the correlation to histopathological findings of the equine palmar foot of lame horses have been described previously, using 0.27 and 1.5 T systems. Compared to these, 3 T systems provide superior spatial resolution and imaging contrast. The aim of our prospective anatomic study was to characterize the imaging anatomy of the navicular region on 3 T MRI in comparison to histopathological findings. We hypothesized that 3 T MRI allows a good visualization of the entire navicular apparatus and reliable measurements of navicular cartilage and cortical bone thickness. Twenty front feet of sound horses were examined using a 3 T MRI system. For histopathological examination, sagittal tissue sections of the navicular bones and adjacent ligaments were prepared. Alterations in magnetic resonance signal were graded for each region and compared to corresponding histological slices. Overall, there was good visualization of the anatomical detail and a very good agreement between MRI and histology for compact bone and spongiosa, good agreement for the fibrocartilage and the distal sesamoidean impar ligament, but only moderate agreement for the hyaline cartilage and the collateral sesamoidean ligament. A comparative measurement of cartilage and cortical bone thickness on magnetic resonance images and histological sections was performed. In MRI, the hyaline cartilage of the articular surface appeared significantly thinner and the fibrocartilage of the flexor surface appeared significantly thicker compared to histology. Findings indicated that MRI at a field strength of 3 T allows reliable depiction of anatomic details of the navicular apparatus.     

Core decompression of the equine navicular bone: an in vivo study in healthy horses

Objective: To determine the physiologic response of the equine navicular bone to core decompression surgery in healthy horses. Study Design: Experimental in vivo study. Materials: Healthy adult horses (n=6). Methods: Core decompression was completed by creating three 2.5‐mm‐diameter drill channels into the navicular bone under arthroscopic control. The venous (P_V), arterial (P_A), articular (P_DIPJ), and intraosseous pressures (IOP) were recorded before and after decompression drilling. Each IOP measurement consisted of a baseline (IOP_B) and a stress test (intramedullary injection of saline solution, IOP_S) recording. Lameness was assessed subjectively and using force plate gait analysis. Fluorochrome bone labeling was performed. Horses were euthanatized at 12 weeks. Navicular bone mineral density (BMD) was measured, and bone histology evaluated. Results: Peak IOP (IOP_max) after stress testing was significantly (P<.05) reduced immediately after core decompression; however, the magnitude of these effects was decreased at 3 and 6 weeks after decompression. A significant (P<.05) correlation existed between IOP_max and BMD. No lameness was observed beyond the first week after surgery. Substantial remodeling and neovascularization was evident adjacent the surgery sites. Conclusion: Navicular bone core decompression surgery reduced IOP_max, and, with the exception of a mild short‐lived lameness, caused no other adverse effects in healthy horses during the 12‐week study period.     

Use of a pressure plate to analyse the toe-heel load redistribution underneath a normal shoe and a shoe with a wide toe in sound warmblood horses at the walk and trot

The objective of this study was to use a pressure plate to quantify the toe-heel load redistribution in the forelimbs of sound warmblood horses with normal shoes and shoes with a wide toe and narrow branches, used empirically in the treatment of superficial digital flexor tendon or suspensory ligament injuries. In a crossover-design study, six horses, randomly shod with normal shoes and shoes with a wide toe, were led over a dynamically calibrated pressure plate to record data from both forelimbs. There were no significant differences between both shoes in the toe-heel index of stance time, peak vertical force and vertical impulse. For the adapted shoe, the peak vertical pressure was slightly lower and was exerted slightly earlier in the stance phase, albeit not significantly. However, the significantly larger toe contact area of the adapted shoe resulted in a significantly lower total vertical pressure in the toe region. Hence, the pressure plate adequately visualised the individual loading of the toe and heel region, and clearly demonstrated the altered pressure distribution underneath the shoe with a wide toe. Although further research on a deformable surface is needed to confirm this hypothesis, the pressure redistribution from the toe to the heels could promote sinking of the heels in arena footing, thereby mimicking the biomechanical effects of a toe wedge and providing a rationale for its application in the treatment of SDFT or SL injuries. The pressure measuring equipment used in this study can offer to the clinician a diagnostic tool for the evaluation of the load distribution underneath the equine hoof and for the fine-tuning of corrective shoeing.     

Magnetic resonance imaging changes of the navicular bursa following navicular bursoscopy in seven horses

Navicular syndrome is a multifactorial disease process in horses with multiple structures in the foot contributing to lameness. Surgical debridement is a treatment option for lesions of the navicular bursa and deep digital flexor tendon. This retrospective case series describes the magnetic resonance imaging (MRI) appearance of the navicular bursa following bursoscopy. Seven horses (three being bilaterally affected) with forelimb lameness isolated to the foot, and pre- and post-operative MRI were included. All limbs had concurrent lesions associated with the deep digital flexor tendon, navicular bone, impar ligament, collateral sesamoidean ligament and/or distal interphalangeal joint. All bursae developed or had progression of proliferative bursal tissue following surgery. At recheck MRI, following rehabilitation protocols, almost all horses had improved to resolved lameness with relatively unchanged concurrent lesions despite the navicular bursa appearance worsening. Outcomes for return to work were poor with only two horses going back to the previous level of work.