Dr. Janicek's current address is Highway 29 Veterinary Hospital, 3500 East State Highway 29, Bertram, Texas 78605, USA.
Reasons for performing study
Fibrotic myopathy can cause incapacitating gait abnormalities. Transection of the fibrotic mass followed by early post operative exercise is the best treatment for fibrotic myopathy. A laser may be used to transect the fibrotic mass. Assessment of the effectiveness of therapies for fibrotic myopathy has been limited to subjective evaluation.
To objectively assess gait abnormalities associated with fibrotic myopathy before and after laser fibrotomy followed by early post operative exercise.
Kinematic evaluation of horses with fibrotic myopathy walking and trotting on a treadmill was used to investigate hindfeet trajectories (n = 8) and lameness (n = 5) before and after laser fibrotomy. Hoof flight trajectory length (HFTL), relative protraction length (%PL), maximum hoof height during swing (MXHH), hoof height at end of protraction (HHpro) and retraction (HHret) were measured and differences between fibrotic myopathy affected and nonaffected limbs were calculated. Lameness was quantified by measuring maximum and minimum pelvic height differences between right and left halves of the stride.
Before surgery the foot of the fibrotic myopathy affected limb had abnormal trajectories characterised as increased HFTL, MXHH and HHpro and decreased %PL and HHret and the 5 horses objectively evaluated for lameness were lame in the fibrotic myopathy affected limb. Immediately after surgery the difference between affected and nonaffected limbs decreased for HFTL, %PL and HHpro. Six to 11 weeks after surgery, the HFTL difference increased but was still smaller than before surgery, which was interpreted as partial recurrence of the gait abnormality; all horses objectively evaluated for lameness were either improved (n = 1) or not lame (n = 4) in the previously affected, operated limb.
Fibrotic myopathy affects the foot flight and leads to asymmetric vertical excursion of the pelvis. Laser fibrotomy followed by early post operative exercise can minimise these abnormalities.
Laser fibrotomy combined with early post operative exercise is a viable therapy for fibrotic myopathy.
Fibrotic myopathy is a well-recognised lameness condition with many cases suspected to be of traumatic origin [1-4]. The site of injury is at the musculotendinous junction of the large stifle flexor muscles, including the semitendinosus, semimembranosus, biceps femoris muscles and, occasionally, the gracilis muscles [1-10]. Simultaneous flexion of the hip and extension of the stifle, such as falling with the affected hindlimb extended underneath the torso and struggling to extract an entrapped hindlimb, direct muscle trauma and intramuscular injections are advocated as causes. Early pain and later scar tissue formation and occasionally soft tissue mineralisation adversely affects hindlimb motion [1-10]. This abnormal motion has been described as sudden interruption of protraction followed by an abrupt downward and backward movement of the foot preceding impact. Kinematic analysis has been performed in one horse with fibrotic myopathy while trotting  but objective assessment of locomotion of horses with fibrotic myopathy at the walk has not been described.
The need for treatment of fibrotic myopathy is open to question since it is reported that some horses perform their function despite the gait deficit and that the condition is not painful [3, 4]. Nevertheless, surgical treatments have been described including complete resection of the abnormal tissue , transection of the fibrotic tissue with a bistoury  and semitendinosus tenotomy near its medial insertion on the tibia [8, 12, 13]. Reports of treatment successes and complications are variable and based upon subjective evaluation [7-10, 12, 13] which is not as sensitive or repeatable as objective kinematic evaluation [14-16]. Evaluator's bias can further compromise the reliability of subjective gait analysis . Complete resection of the fibrotic tissue is no longer recommended because of the high incidence of surgical site dehiscence and recurrence of the gait abnormality . The less invasive semitendinosus insertional tenotomy has a low complication rate  but in our practice has not provided long-term improvement. A surgical procedure to transect the fibrotic tissue with a bistoury with the horse standing followed by early post operative exercise has also previously been reported . Based on subjective evaluation of the gait, the average short-term improvement was 75% and only 66% of the horses maintained their initial level of improvement after an average follow-up time of 27 months. Therefore, we modified the standing myotomy procedure by utilising a laser to transect the fibrotic mass with the aim of minimising bleeding, post surgical inflammation and adhesion formation. Early post surgical exercise to mitigate recurrence of the gait abnormality was also encouraged. The aim of this study was to objectively assess with high speed, computer-assisted kinematic gait analysis the gait abnormalities associated with fibrotic myopathy before and after laser fibrotomy. Our hypotheses were: 1) At the walk the cranial phase of the stride of the limb affected with fibrotic myopathy is shortened and the foot drops abruptly to the ground at the end of protraction, 2) at the trot there is an imbalance of the vertical motion of the pelvis suggestive of lameness of the limb affected with fibrotic myopathy and 3) in horses with fibrotic myopathy, laser transection of the fibrotic mass followed by early post operative exercise would minimise the difference of the motion between the affected and the contralateral limbs and eliminate the imbalance of the vertical motion of the pelvis.
Materials and methods
Eight horses (age 8–20 years, mean 14.1; weight 459–573 kg, mean 495; 2 geldings and 6 mares; 3 Quarter Horses, one Paint, one Appaloosa, one Arabian, one Tennessee Walking Horse and one Missouri Fox Trotter) with fibrotic myopathy presented to the College of Veterinary Medicine, University of Missouri between June 2006 and August 2007 for evaluation of lameness. Affected horses were enrolled if no surgical treatment had been previously attempted and after owners consented for surgery and kinematic evaluation. This study was approved by the Institutional Animal Care and Use Committee. Diagnoses were based solely on history, clinical observation and limb palpation. In 7 horses clinical signs were first seen 1.5–12 months prior to presentation. In one horse the time course of development was unknown. Five horses were used for pleasure riding, one for show, one for barrel racing and one for cutting competition. In 3 horses the precipitating event was a kick from another horse. In 2 cases falls on ice were thought to be the cause. In 3 cases the precipitating event was unknown. Six cases were affected in the left and 2 in the right hindlimb. Ultrasonographic examinations were not performed.
Gait analyses to quantify hindlimb movement were performed while the horses were walking on a treadmill (Sato Equine Treadmill)1 at 3 different times, 1) before surgery, 2) 1–2 days after surgery and 3) 6–11 weeks after surgery. Five horses were also trotted on the treadmill for lameness evaluation the day before surgery and 6–11 weeks after surgery. No evaluation at the trot was performed 1–2 days after surgery for fear of promoting surgical site dehiscence. Treadmill speed was determined individually so each horse could move at the desired gait and comfortably maintain position on the treadmill.
Reflective spheres (2.5 cm in diameter) were placed on the dorsum of the pelvis between the sacral tuberosities (TS), on the lateral aspect of the right hindlimb hoof and fetlock and on the medial aspect of the left hindlimb hoof. The marker on the right fetlock was used only to differentiate right from left hoof during data processing. Three strobe-activated, visible red-detecting cameras, positioned for a right-sided field-of-view, from 3 different angles, captured motion at 120 frames/s. Using a motion analysis system (Vicon 250)2, 3-dimensional position of each marker was recorded and tracked for 30 s resulting in collection of at least 25 strides at the walk and 35 strides at the trot.
Surgical procedure and post operative care
After sedation with detomidine hydrochloride (0.02 mg/kg bwt i.v.) and butorphanol tartate (0.01 mg/kg bwt i.v.), the tail was isolated and retracted away from the proposed surgery site using gauze bandage. The region of the semimembranosus/semitendinosus musculotendinous junction was clipped and aseptically prepared. Approximately 100 ml of 2% mepivicaine hydrochloride was injected in an inverted U-shaped block proximal to and around the proposed incision site. A 5–8 cm vertical incision was made in the skin over the fibrotic area, just proximal to the myotendinous junction, with a scalpel. Haemostasis was controlled by contact application of Nd:YAG laser using a nonsculpted bare fibre (diameter, 1.05 mm) with power set at 20 watts. Blunt dissection of the subcutaneous tissue was performed to expose the fibrotic mass. Because of the dense fibrosis and adhesions to the subcutaneous tissues over the fibrotic area, differentiation of the specific muscle(s) involved was difficult. However, in all cases, both semitendinosus and semimembranosus muscles were thought to be involved. Using 2 large Carmalt haemostats in a blunt tunnelling technique, the entire fibrous mass was encircled and separated from surrounding apparently normal tissue. The fibrotic mass was transected with a contact fibre beginning at 20 watts of power and adjusting upward to 50 watts as necessary to allow progression without excessive heating of surrounding tissue. The area was flushed with cold, sterile saline to cool surrounding normal tissues and to remove charred residue. Transection of the fibrotic mass required inclusion of medial fascia separating the semimembranosus from the gracilis muscle and lateral fascia separating the semitendinosus muscle from the biceps femoris muscle. Attention was given to preserving normal muscle deep to the fibrotic mass.
After transection, a 1.5 cm penrose drain was inserted, exiting the surgical wound through stab incisions immediately proximal and distal. The drain was sutured to the skin around the stab incisions. The primary incision was closed in 2 layers, the subcutaneous layer with 2-0 polydioxanone (PDS)3 in a simple continuous pattern and the skin with No. 2 PDS in a simple interrupted pattern. No stents were used. Time required for completion of the whole procedure including administering sedatives, clipping and scrubbing the surgical site, injecting local anaesthetics and the surgical procedure itself was about 1.5 h.
Perioperative treatment included trimethoprim-sulfadiazine (30 mg/kg bwt per os b.i.d. for 14 days), phenylbutazone (1 g per os b.i.d. for 2 days, followed by 1 g per os s.i.d. for 3 days) and daily application of petrolatum to the skin distal to the drain exit site. All horses were discharged from the hospital within 2 days after surgery. The drain was removed in 2–4 days and the sutures in 11–13 days. Post operative exercise protocol was started one day after surgery and consisted of 2–3 sessions of controlled hand walking daily for 3 weeks (5–10 min sessions during the first week followed by gradual increase by 5–10 min each week). After the initial 3 weeks of controlled hand walking, exercise or unlimited free turnout commenced at the discretion of the owner.
The owners were asked to bring the horses back to the clinic 45–60 days after surgery.
Attempts were made to contact all owners of horses 4–5 years after surgery to assess post operative complications, hindlimb lameness and to gauge owner satisfaction.
Analysis of hindlimb hoof trajectories
By convention, the x-direction was caudal to cranial and the z-direction ventral to dorsal in the sagittal plane. Movement in the y-direction (right to left in the frontal plane) was not analysed. All marker trajectory signals were separated into single stride units using hoof-z position as indicator of hoof impact. Hoof trajectory was divided into protraction and retraction on the basis of relative position to the TS marker x-position. The following hindlimb hoof trajectory variables were calculated for each stride; 1) Hoof flight trajectory length (HFTL = maximum hoof x minus minimum hoof x, mm); 2) Protraction length as a percentage of HFTL (%PL = (maximum hoof x minus TS x-position at same instant)/(HFTL) X 100); 3) maximum hoof height during flight (MXHH = maximum hoof z, mm); 4) hoof height at the end of protraction (HHpro = hoof z at maximum hoof x, mm); 5) and hoof height at the end of retraction (HHret = hoof z at minimum hoof x, mm).
Mean ± s.d. of the hoof trajectory variables from all strides were calculated for each hindlimb. Differences in means between the affected and unaffected limbs were then calculated. All variable calculations were performed using custom-written macros and script in commercial software (Microsoft Excel4 and SAS5).
Analysis of hindlimb lameness
Evaluation of hindlimb lameness was performed as previously described . Briefly, vertical position of the pelvis and right hindlimb hoof were tracked. The vertical pelvic position signal was decomposed into first (lameness component) and second (natural vertical movement component) harmonics and a nonperiodic component. After removing the nonperiodic component, harmonics were summed to produce gross vertical pelvic movement. Using the vertical right hindlimb hoof signal as the differentiator between right and left stance phases of the stride, stride-by-stride local maximum and minimum dorsal pelvic positions were determined and assigned to right and left phases of the stride. Differences in local maximum positions (diffmaxpelvis) at the end of right and left stance phases of the stride indicated relative amount of hindlimb propulsion with positive values indicating deficient right hindlimb propulsion (+diffmaxpelvis) and negative values indicating deficient left hindlimb propulsion (-diffmaxpelvis). Differences in local minimum positions (diffminpelvis) during right and left stance phases of the stride indicated relative amount of impact on each hindlimb with positive values indicating deficient right hindlimb impact (+diffminpelvis) and negative values indicating deficient left hindlimb impact (-diffminpelvis). Diffmaxpelvis and diffminpelvis were determined for each stride and the mean ± s.d. over all strides calculated for each trial. Thresholds for diffmaxpelvis and diffminpelvis between sound and lame, estimated from previous studies of horses with induced lameness trotting on the treadmill, were ± 3 mm [18, 19]. These calculations were performed using custom-written script (MATLAB 7.0.1)6.
Effect of surgery on hindlimb hoof trajectory variables (before vs. 1–2 days and 6–11 weeks after surgery) was investigated with Friedman's test. Effect of surgery on hindlimb lameness variables (before vs. 6–11 weeks after surgery) was investigated with the Wilcoxon signed ranks test. Commercial software (StatsDirect)7 was used for statistical analyses.
Hoof trajectories before surgery
Evaluation of composite hoof trajectories of the affected hindlimb revealed a variety of shapes (Fig 1). The most characteristic trajectory feature in the affected limb, seen in all horses, was abrupt downward movement of the hoof at the end of protraction. In all but one horse this abrupt downward movement was preceded by rising of the hoof until it reached a local maximum height immediately before impact. In 5 horses this rising of the hoof before impact was dramatic with relatively low hoof height during early swing. In 5 cases there was decreased protraction in the affected limb (Horses 2, 3, 5–7) compared with the unaffected limb but the most consistent and apparent difference was increased retraction in the affected limb (all horses except Horse 1), so overall HFTL was longer in the affected than in the unaffected limb (Table 1). Affected limbs also displayed higher maximum hoof height (MXHH) during swing, higher hoof height at end of protraction (HHpro), and lower hoof height at the end of retraction (HHret) compared with the opposite, unaffected limb.
Table 1. Hindlimb hoof trajectory variables in 8 horses with fibrotic myopathy walking on a treadmill before and after laser fibrotomy. Median and range of differences between affected and contralateral (unaffected) limbs are displayed. Horses were evaluated one day before surgery, 1–2 days after surgery and 6–11 weeks after surgery
Different superscripts indicate significance (P<0.05).
HFTL = Hoof flight trajectory length (maximum hoof x minus minimum hoof x). %PL = Protraction length as a percentage of HFTL. MXHH = Maximum hoof height (maximum hoof z). HHpro = hoof height at the end of protraction (hoof z at maximum hoof x). HHret = hoof height at the end of retraction (hoof z at minimum hoof x).
Hindlimb foot trajectories (before vs. after surgery)
Immediately after surgery, the abrupt downward and backward movement of the foot preceding impact was visibly reduced (Fig 1, Table 1). Differences in HFTL (P<0.001) and %PL (P = 0.026) were significantly decreased after surgery. Difference in hoof height at the end of protraction (HHpro) was significantly decreased (P<0.001), but differences in overall maximum hoof height (MXHH) and HHret were unchanged after surgery.
Seven horses returned for kinematic re-evaluation between 41 and 78 (approximately 6–11 weeks) days after surgery. In all cases, the surgical wound was completely healed. In 6 horses a mild hindlimb foot flight abnormality characteristic of fibrotic myopathy was noted but subjective consensus was that this abnormality was visibly improved. Kinematic evaluation indicated that, although median difference in HFTL was significantly smaller than before surgery, it was significantly increased from 1–2 weeks after surgery (Table 1). However, differences in median %PL and median HHpro that were seen 1–2 days after surgery were preserved 6–11 weeks after surgery (Table 1).
Hindlimb lameness (before vs. after surgery)
Before surgery all 5 horses subjected to objective evaluation at the trot were lame in the affected hindlimb (Table 2). Maxdiffpelvis and mindiffpelvis were above the estimated threshold in 4 of the 5 horses. In one horse only maxdiffpelvis was above the threshold. In 3 horses maxdiffpelvis was greater than mindiffpelvis and in 2 horses mindiffpelvis was greater than maxdiffpelvis. After surgery 4 out of 5 horses were lame: 3 horses (horses 4, 6, 8) were lame in the opposite, unoperated limb; Horse 5 was mildly lame in the operated limb with maxdiffpelvis (5.6 mm) only slightly above the threshold and one-third the value before surgery (16.7 mm) and Horse 7 was sound in both hindlimbs after surgery with both mean maxdiffpelvis (1.6 mm) and mean mindiffpelvis (0.9 mm) below the estimated threshold.
Table 2. Kinematic evaluation of hindlimb lameness in 5 horses with fibrotic myopathy trotting on a treadmill before and 6–11 weeks after laser transection of the fibrotic tissue
Maxdiffpelvis ± s.d. (mm)
Mindiffpelvis ± s.d. (mm)
Limb with fibrotic myopathy
Maxdiffpelvis = Difference in maximum pelvic height after push-off between left and right hindlimb strides; Mindiffpelvis = Difference in minimum pelvic height during stance between left and right hindlimbs. Estimated threshold value of between ‘lame’ and ‘sound’ is ± 3 mm. Within each row, different superscripts indicate significance (P<0.05).
-16.3 ± 3.2
-19.1 ± 4.1
11.1 ± 4.5
5.5 ± 4.0
16.7 ± 6.6
1.3 ± 4.6
5.6 ± 5.0
-1.0 ± 4.2
-10.5 ± 18.7
-27.9 ± 25.0
11.9 ± 11.2
17.2 ± 16.0
-37.7 ± 5.8
-10.0 ± 4.0
1.6 ± 13.0
0.9 ± 4.3
-8.7 ± 4.6
-3.5 ± 5.4
11.0 ± 9.3
8.3 ± 8.4
Long-term follow-up information 4–5 years after surgery was obtained from 6 owners (Horses 3–8). Four owners reported that abnormalities in the horse's gait could not be seen anymore and the horses were able to return to full use. The owner of Horse 5 reported partial improvement and that the horse was being used in pleasure riding. The owner of Horse 7 reported that surgery had no benefit and the horse could not be used for riding. The most severe complication reported by the owners was drainage from the surgical wound, which was observed in 3 horses. Swelling for a few weeks after surgery was also mentioned by 2 horse owners. Only one owner (Horse 7) was disappointed with the results of surgery.
This study presents objective information about gait abnormalities in horses with fibrotic myopathy before and after laser transection of the fibrotic mass followed by early post operative exercise and introduces laser transection of the fibrotic mass (fibrotomy) as an alternative treatment for fibrotic myopathy in horses. This procedure improved limb movement and lameness. Wound complications frequently reported with other surgical procedures for fibrotic myopathy were minimised. The laser procedure was performed standing under sedation and local anaesthesia with simple closure of the surgical wound.
Other surgical techniques for treatment of fibrotic myopathy have been described. Seventeen horses with fibrotic myopathy involving only the semitendinosus and one horse with additional gracilis muscle involvement were treated under general anaesthesia by completely excising the fibrotic mass . Wound complications including dehiscence were seen. One difficult anaesthetic recovery (limb trapped in full extension underneath the abdomen) and post surgical abnormality (hyperextension of the stifle) was reported. Of the 9 horses available for follow-up, one was free of lameness, with 7 partial improvements and one with no improvement. The authors concluded that less radical excision combined with physical therapy might be beneficial in reducing reformation and contraction of the fibrotic tissue and recurrence of the abnormal gait.
As an alternative to fibrotic mass excision, a semitendinosus tenotomy at its tibial insertion has been used to treat horses with fibrotic myopathy . This procedure is easy to perform and is less invasive. Although it is feasible to perform this surgery standing because of the medial location of the approach high on the hindlimb, this procedure is usually performed under general anaesthesia. In 3 horses treated with semitendinosus tenotomy at its medial insertion alone and in one horse treated with tenotomy at the tibial insertion and tenotomy of the remaining tendon, recovery was considered equal to partial myotenectomy. However, 2 of these cases were less than one year of age and had suspected ‘congenital’ fibrotic myopathy with characteristic gait abnormality but no recognisable fibrotic mass. In 3 other cases described in subsequent reports, semitendinosus tenotomy at its tibial insertion resulted in either improvement (2 cases) or resolution of the gait abnormality [12, 13]. In our experience, the semitendinosus tenotomy technique at its medial insertion has not been sufficiently effective for fully correcting clinical signs of fibrotic myopathy. Unsatisfactory results obtained with semitendinous tenotomy may be attributable to inappropriate case selection (e.g. horses with extensive fibrosis affecting stifle flexor muscles other than the semitendinous muscle). The effectiveness of semitendinosus tenotomy at its medial insertion, when pathology includes other stifle flexor muscles, has not been reported. Furthermore, in the reported cases of semitendinous tenotomy, the outcome was considered successful based exclusively on subjective evaluation.
The greatest number of patients treated surgically for fibrotic myopathy was reported by Magee and Vatistas . Thirty-nine horses with fibrotic myopathy were treated by standing semitendinosus myotomy using a bistoury and then started early (36 h after surgery) on ‘a controlled exercise programme’. Based on subjective evaluation, the authors concluded that about one-third of the cases had initial complete resolution of the gait abnormality and 40% had 75% improvement. Two-thirds of the cases maintained this initial improvement long-term (average follow-up time, 27 months) and about one-third had some recurrence, with only 10% completely regressing to the severity seen before surgery. Owners were satisfied with results in 94% of the cases. The authors concluded that standing myotomy was easy to perform and with early post operative exercise gave satisfactory results.
With both the procedure described in the current study and the technique reported by Magee and Vatistas (1998), the mass of fibrotic tissue was transected. Advantages of laser transection over transection with a simple blade include more controlled and complete haemostasis  which improves visualisation of the fibrotic mass and facilitates its differentiation from deeper normal muscle. The sealing of small vessels during the transection may also decrease early post operative inflammation and swelling and minimise the amount of the scar that will reform after surgery. Alternatively a CO2 laser could have been used instead of ND:YAG laser. A CO2 laser minimises production of tissue char and minimises collateral heating of tissue but would provide less haemostasis for this particular application . With early and sufficient post operative exercise, decreased incidence of restrictive fibrosis helped to prevent reoccurrence of the gait abnormality. Unfortunately it is not possible to compare the early post operative exercise protocol of the present study with that of the article reporting bistoury transection of the fibrotic tissue since details of the exercise protocol were not provided in the previous report .
Ultrasonographic examination was not performed in the present study but it has been suggested that this may help determine the extent of stifle flexor muscle involvement [3, 4]. In all of the cases described in the current report it was found during surgery that the fibrotic mass extended into the muscle mass of the semimembranosus. Involvement of the dermis and subcutaneous tissues as well as adhesions between all of the stifle flexors were common. Complete transection of the fibrotic mass necessitated inclusion of the fascia separating the stifle flexor muscles. Simple tenotomy of the semitendinosus muscle at its tibial insertion  is unlikely to be effective with such widespread formation of restrictive fibrosis.
Loss of the initial gait improvement seen immediately after surgery had been reported before in horses with fibrotic myopathy subjected to transection of the fibrotic tissue  and was objectively demonstrated in the present study. It is likely that reformation and contraction of the fibrotic tissue was the cause of the partial recurrence of gait abnormalities. Complete preoperative assessment of the extent of the fibrotic tissue using ultrasonography may help to plan the surgical procedure and assure that all fibrotic tissue is transected; better understanding of the anatomy of the fibrotic tissue may also contribute to minimise tissue damage during the surgical procedure. It is tempting to make additional modifications to the therapeutic protocol with the aim of reducing the rate of post operative recurrence of gait abnormalities associated with fibrotic myopathy. The use of drugs to modulate the healing process and minimise scar tissue formation and contraction (e.g. corticosteroids, antimetabolites) may be beneficial. However, this kind of intervention may lead to complications such as delayed healing of the surgical wound and infection . Barriers developed for prevention of peritoneal adhesions such as an absorbable hyaluronate-carboxymethylcellulose membrane  may minimise reformation of fibrotic tissue if placed in the gap created in the affected muscle(s). In human surgery, absorbable barriers have been used to minimise formation of scar tissue between disc remnants, muscles, bones and nerve roots and ultimately prevent post operative pain . Similarly however to corticosteroid therapy, the use of barriers has potential to compromise healing and increase the risk of infection . Changes in the exercise protocol with the aim of promoting more stretching of the stifle flexor muscles (e.g. introduction of hamstring passive stretching) may contribute to further elongate the musculotendinous units. Although this approach has been recommended for horses with fibrotic myopathy , there is no report of its use in clinical cases.
Using the objective measurement of vertical pelvic movement as the indicator of lameness, at presentation all horses with fibrotic myopathy displayed hindlimb lameness in the affected limb. During the stance of the limb affected with fibrotic myopathy, reduced upward movement of the pelvis was seen in all horses and reduced downward movement of the pelvis was seen in 4 of the 5 horses. In the 5 horses evaluated objectively for lameness 6–11 weeks post surgery, lameness in the treated limb was either no longer present (4) or improved (1). Curiously, mild lameness developed in the opposite hindlimb in 3 horses. Increased weightbearing on the opposite limb for long periods of time before surgical correction and during convalescence may have been contributory for post operative lameness of the contralateral limb. It is also possible that transecting the stifle flexor muscles with subsequent increase in functional length of the stifle flexors during recovery created this asymmetric pelvic movement. Since 5 of the 6 cases with long-term follow-up were able to return to normal function, this mild asymmetry of vertical pelvic movement either resolved or was not noticed by the owner.
In the single, previously reported case studied objectively with high-speed kinematics, which was only evaluated at trot  and in most subjective descriptions [1-4], fibrotic myopathy caused shortened hindlimb protraction. In the current study 5 horses did indeed display shortened relative protraction of the affected hindlimb. Furthermore, limb protraction increased after surgery. However, a more consistent gait abnormality seen in this study that has not been previously reported in horses with fibrotic myopathy was increased retraction of the affected limb. The increased limb retraction effectively increased the length of the affected limb's hoof trajectory. Prolonged or delayed weightbearing at the end of the stance phase of the stride may be caused by pain in the stifle flexors during eccentric tension. Prolonging the end of stance may contribute to spread out the total energy absorbed in the stifle flexors, essentially decreasing peak tensile force. Total stance duration has been shown in several reports to increase, not decrease, in horses with mild to moderate weightbearing lameness, illustrating this same phenomenon [25-30].
Two owners reported at long-term (years) follow-up that the abnormal gait had returned. In Horse 5 at 6 weeks following surgery, hindlimb trajectory did appear to return to presurgical shape but hindlimb lameness was decreased. This owner was happy with the results and returned the horse to its intended function. In Horse 7, the hindlimb hoof trajectory abnormality only partially returned at 6 weeks after surgery and hindlimb lameness in this horse was completely eliminated. This owner was dissatisfied with the surgery because the horse could not be returned to its prior use. Further attempted follow-up was not successful and it can only be assumed that the cause of the dissatisfaction in this case was associated with the horse's fibrotic myopathy.
It can be concluded that, at the walk, the foot of the fibrotic myopathy affected limb has an increased height at the end of protraction and an increased flight trajectory length. At the trot, the vertical excursion of the pelvis is also affected and push-off lameness on the affected side is the most consistent finding. It can also be concluded that laser transection of the fibrotic mass followed by early post operative exercise is a viable approach to minimise foot flight abnormalities and to eliminate lameness associated with fibrotic myopathy but should not be expected to produce complete and permanent remission of gait abnormalities in all horses.
Authors’ declaration of interests
No competing interests have been declared.
Source of funding
Supported by the E. Paige Laurie Endowed Program in Equine Lameness, University of Missouri, College of Veterinary Medicine, Columbia, Missouri, USA.
Thanks to Dr. Marta Bianchesi, Dr. Juliana Amorim and Becky Elias for their help with data compilation.
All authors effectively contributed to the study: J. Janicek - study design, data collection and study execution, data analysis and interpretation and preparation of the manuscript. M.A.F. Lopes - data analysis and interpretation and preparation of the manuscript. K.G. Keegan - study design, data collection and study execution, data analysis and interpretation and preparation of the manuscript. D.A. Wilson - data collection and study execution, data analysis and interpretation and preparation of the manuscript. S. Reed - data analysis and interpretation and preparation of the manuscript.