Introduction
Shoulder instability has been classically described as a frequent cause of forelimb lameness in dogs.1,2 This condition may occur as a congenital soft tissue laxity that progresses to the point of subluxation and clinical signs or results in medial shoulder luxation (MSL).1,3 Medial shoulder luxation occurs more frequently than luxations in other directions, and this predominance is reportedly associated with damage to the medial glenohumeral ligament, congenital shoulder instability, and hypoplasia or dysplasia of the glenohumeral joint.1,3,4 The etiologies of these diseases are not fully understood, and their relationships remain unclear. Glenoid dysplasia commonly presents congenitally or during early stages of life in toy breeds and typically results in MSL.5 A recent report3 on medial shoulder instability (MSI) indicated that Miniature/Toy Poodles were > 10 times overrepresented in the MSI populations compared to other breeds.
Several surgical treatments have been reported for MSL and MSI.6–9 Experimentally created MSI, made by severing the tendon of insertion of the subscapularis muscle and the medial joint capsule, was treated with medial transposition of the tendon of the biceps brachii muscle or the tendon of the supraspinatus muscle.10 While these methods provide medial stability, it is important to note that they compromise tissues that contribute to cranial or lateral joint stability and affect physiological joint function. Other methods such as prosthetic ligaments, arthrodesis, and excision arthroplasty require extensive soft tissue dissection.6–9
The technique used in this study is a modification of a previously reported surgical method.11 Present technique employs an antiluxation pin inserted from caudal to the supraspinatus muscle tendinous insertion of the greater tubercle into the humerus distally. The protruding portion of the pin lateral to the supraspinatus muscle tendon functions as an active stabilizer preventing medial luxation without compromising tissues contributing joint stability and physiological joint function. The purpose of this report is to describe the clinical and diagnostic findings, surgical procedures, and clinical course of MSL in toy-breed dogs treated with the antiluxation pin. Our hypothesis was that the antiluxation pin would prevent reluxation of the shoulder joint and improve lameness scores (LMS).
Methods
Data collection
Medical records of toy-breed dogs presented to the Aikawa Veterinary Medical Center (referral hospital) with MSL and treated with the antiluxation pin between 2017 and 2022 were included and reviewed. Clinical data obtained from the medical records included signalment, cause of luxation, preoperative and follow-up LMS, outcome, and radiographic and/or CT findings. Lameness scores were used for gait assessment (0, normal gait pattern; 1, mild intermittent lameness; 2, moderate lameness, normal stride length, partial weight-bearing; 3, moderate lameness, shortened stride length, partial weight-bearing; 4, severe lameness with toe-touching and minimal limb use; and 5, non–weight-bearing lameness).12 Owner consent was obtained prior to the dogs’ participation in the study.
Anesthetic protocol and postoperative care
The dogs underwent preoxygenation for 10 minutes before induction with atropine (0.01 mg/kg, IV) and propofol (4 to 7 mg/kg, IV to effect). Anesthesia was maintained with isoflurane (1.5% to 2.0%) in oxygen using a ventilator. Analgesia was maintained using brachial plexus block (bupivacaine, 1 mg/kg) and morphine (0.5 mg/kg, IM). Cefazolin sodium (22 mg/kg, IV) was administered 30 minutes before skin incision. Postoperative analgesia was maintained using morphine (0.5 mg/kg, IM) for 24 to 48 hours, as needed. Robenacoxib was administered PO for 3 to 5 days postoperatively. No strict postoperative exercise restrictions were implemented, and bandages were not used. Return to normal activity took place progressively over a period of 2 to 6 weeks. Exercise was increased when dogs showed no sign of lameness.
Surgical procedure
Each dog was placed in lateral recumbency with the affected limb upward. An assistant maintained the scapula in an elevated and abducted position by pulling a towel clamp attached to the acromion. The surgeon externally rotated the humerus and pushed the humeral head medially to cause MSL and manipulated the position at which the shoulder joint was easily luxated and reduced. An arthrotomy was performed between the supraspinatus and infraspinatus muscles on the lateral side of the shoulder joint, allowing access to the caudal aspect of the greater tubercle of the humerus. The threaded pin (IMEX Veterinary Inc) was inserted from 0.5 to 1.0 cm caudal to the supraspinatus muscle tendinous insertion of the greater tubercle into the humerus distally, contacting its shaft to the lateral belly of the distal musculotendinous region when the shoulder joint was in the middle shoulder range of motion (approx 110°). The pin diameter was determined as approximately 30% of the diameter of the humeral shaft.
The proper pin insertion point and angle were assessed by a “dynamic” evaluation (Supplementary Video S1) to accommodate the compromised intrinsic stability in each case. The pin was placed in a position where the pin engaged the distal supraspinatus musculotendinous region and prevented MSL when the load of luxation was applied. The effect of the antiluxation pin was stronger when it was placed more caudally or medially and/or the proximal part of the pin was directed medially to the humerus (and the distal part of the pin directed laterally). The maneuver was tested by holding the pin in the trial position manually. The surgeon confirmed that the pin countered the load of luxation at all ranges of shoulder joint motion with no excessive pressure to the supraspinatus muscle tendon. Then the pin was inserted 2 to 3 cm distally into the humerus and was cut approximately 1 cm from the bone surface, and the surgical site was closed (Figure 1).
Schema of the antiluxation pin inserted lateral to the supraspinatus muscle tendon.
Citation: Journal of the American Veterinary Medical Association 262, 9; 10.2460/javma.23.11.0652
Outcome assessments
Limb function was subjectively evaluated by LMS. Assessment of patient progress was evaluated at recheck appointments, typically at 1, 2, and 6 months postoperatively, and then annually. This assessment included clinical examinations and orthogonal radiographs of the shoulder. For dogs that did not have annual follow-up data gained by the recheck examination, the final LMS was acquired by a follow-up telephone interview with the owner.
Statistical analysis
The Wilcoxon signed rank sum test was performed to compare the preoperative LMS with those at the final follow-up. This was considered significant at P < .05.
Results
Signalment and presentation
Twenty limbs of 18 dogs met the inclusion criteria. Breeds included Toy Poodle (n = 17) and Yorkshire Terrier (1). The median age and weight were 10 years (range, 1 to 14 years) and 3.4 kg (range, 0.8 to 5.1 kg), respectively. Six dogs were male (4 castrated) and 12 were female (9 spayed). The affected limbs included 8 left and 12 right limbs. All but 1 dog that was injured after falling from a table had no obvious history of trauma. Seventeen limbs demonstrated acute-onset lameness, while 3 limbs exhibited chronic, intermittent lameness. The periods from onset to admission were as follows: 0 to 3 days (7 limbs), 4 to 10 days (2 limbs), 11 to 30 days (7 limbs), and 1 to 6 months (4 limbs). Six limbs underwent closed reduction followed by a Velpeau sling placement at other hospitals prior to presentation.
Diagnosis
The preoperative LMS were 1 (1 limb), 2 (1 limb), 3 (1 limb), 4 (4 limbs), and 5 (13 limbs). All dogs underwent preoperative radiographic evaluation. The most common radiographic and/or CT finding was medial luxation, observed in 17 limbs. Other findings included degenerative joint disease (9 limbs), subchondral bone sclerosis of the humeral head (4 limbs), shallow glenoid cavity (2 limbs), flattened humeral head (2 limbs), and subluxation (1 limb). One limb showed no abnormalities on radiography. Medial luxation was confirmed in all limbs using radiography or palpation under general anesthesia (Figure 2).
An 8-year-old 3.3-kg Toy Poodle. Postoperative craniocaudal (left image) and mediolateral (right image) radiographs showing medial shoulder luxation.
Citation: Journal of the American Veterinary Medical Association 262, 9; 10.2460/javma.23.11.0652
Surgery
Two dogs underwent bilateral surgery: one in a staged manner and the other as a 1-stage bilateral procedure. The pin diameters were 1.6 mm (n = 2; body weight range, 0.8 to 1.5 kg), 2.0 mm (13; range, 1.4 to 4.5 kg), and 2.4 mm (5; range, 3.0 to 5.1 kg). The operative times were between 15 and 30 minutes (Figures 3 and 4).
Postoperative craniocaudal (left image) and mediolateral (right image) radiographs of a 10-year-old 3.0-kg Toy Poodle. Typical antiluxation pin placement of the caudal aspect of the greater tubercle of the humerus. The pin diameter is 2.3 mm.
Citation: Journal of the American Veterinary Medical Association 262, 9; 10.2460/javma.23.11.0652
Postoperative craniocaudal (left image) and mediolateral (right image) radiographs of an 11-year-old 1.5-kg Toy Poodle. The pin placement with stronger antiluxation effect. Note the pin is placed more caudally to the greater tubercle of the humerus. The pin diameter is 1.6 mm.
Citation: Journal of the American Veterinary Medical Association 262, 9; 10.2460/javma.23.11.0652
Outcome
During the follow-up period, 17 limbs remained unluxated. Reluxation occurred in 3 limbs (1, 4, and 30 days postoperation). Postoperative lameness, with LMS ≥ 2, persisted or recurred in 4 limbs (LMS 2, 1 limb; LMS 3, 3 limbs). No postoperative seroma or skin ulceration developed in the dogs. Additionally, there were no cases of pin loosening or migration. Out of the 3 limbs with reluxation, 1 limb had a good outcome after the revision surgery, in which the original pin was replaced by 2 parallel pins. These pins were inserted from more caudal points and directed more laterally to the humerus to increase the antiluxation effect. Another limb was deemed unsuitable for revision surgery due to severe degenerative joint disease, leading to the removal of the pin. The remaining limb received no further treatment due to systemic disease.
Of the 3 limbs with persistent postoperative lameness, 1 limb’s lameness and pain resolved after 2 revision surgeries to adjust the pin orientation on days 30 and 72 after initial surgery. The original pin was replaced by one inserted from more cranial points, directed more caudally to reduce the antiluxation effect on extension in the first revision surgery. Then, the pin of the first revision was replaced by one inserted from the same point directed more caudomedially in the second revision surgery to further reduce the antiluxation effect. The other 2 limbs did not receive additional treatment. A relapse of lameness (LMS 3) after the initial improvement was observed in 1 limb, 17 months postoperation.
The LMS for 18 unluxated limbs after primary or revision surgery were 2 (1 limb), 3 (1 limb), 4 (4 limbs), and 5 (12 limbs) preoperatively, and 0 (14 limbs), 1 (1 limb), and 3 (3 limbs) at the final follow-up. The median follow-up period was 18.5 months (range, 1 to 63 months). The LMS at the final follow-up for the 18 limbs (mean, 0.5) was significantly lower than the preoperative LMS (mean, 4.5) (P < .01).
Discussion
The antiluxation pin of the humerus, placed into the caudal aspect of the greater tubercle, successfully prevented the recurrence of MSL and maintained a good long-term function in most cases. No implant failures were observed.
The group of patients with MSL in this study predominantly consisted of Toy Poodles, as previously reported.4 The age of onset varied, with a median age of 10 years. In dogs, shoulder joint stability is dependent on the complex interaction between the active and passive stabilizers of the joint. Active stabilizers comprise the periarticular cuff muscles, and passive stabilizers comprise the joint capsule and glenohumeral ligaments.13 Both active and passive mechanisms function in unison, with the active stabilizers being more important when humeral head displacement is small and the passive stabilizers playing a more important role when humeral head displacement is greater.13,14 The supraspinatus muscle is one of the rotator cuff muscles of the shoulder joint, originating cranial to the spine of the scapula. Its distal musculotendinous region passes craniolaterally through the shoulder joint and inserts on the greater tubercle, contributing to the active craniolateral stabilization of the shoulder joint. In cases without congenital shoulder abnormalities, shoulder instability can result from atrophy of the periarticular musculature with age, chronic damage to shoulder-joint–supporting structures, and degenerative joint disease caused by chronic instability.4,7 Low-grade dysplasia of the shoulder joint promotes these acquired changes.
The antiluxation pin, which is inserted into the caudal aspect of the greater tubercle and positioned on the lateral side of the supraspinatus muscle, acts as an active stabilizer by applying pressure to the supraspinatus muscle tendon when the humeral head moves medially due to subluxation. This procedure is not suitable for cases of lateral, cranial, or caudal luxation.
Intraoperative “dynamic” evaluation involves placing the humerus in a position that allows easy manipulation of luxation and reduction of the shoulder joint. It allows for the evaluation of the position and angle of the antiluxation pin that prevents medial luxation against the force of luxation. Signs of instability are reportedly recognized using a craniocaudal or mediolateral drawer sign or both.1 In our study, medial instability and attempted medial luxation were assessed as the caudal humeral head sliding medially, with the greater tubercle where the supraspinatus muscle attaches as the fulcrum.15
The antiluxation effect of the pin depends on its insertion position and angle. Inadequate antiluxation effect increases the risk of reluxation. Stronger antiluxation effect is needed for cases with severely compromised intrinsic stability. Excessive antiluxation effect to the distal supraspinatus musculotendinous region may cause persistent postoperative discomfort or lameness. Since the supraspinatus muscle functions as a stabilizer, it is essential that it has sufficient strength without severe damage or atrophy. The craniocaudal angle of the pin protruding from the bone surface determines the extension-flexion angle of the shoulder joint at which the antiluxation effect is maximized. We evaluated the optimal position and angle of the pin for each case intraoperatively, which counters the external forces of medial luxation yet does not exert excessive pressure on the supraspinatus muscle within the shoulder range of motion (Figures 3 and 4). Further research is required for the objective measurement and analysis of the position and angle of the pin.
Compared to the use of screws, the threaded pins are advantageous in that dynamic evaluation and pin placement can be performed in 1 motion and the cutoff point of the pin can be determined after placement. The technique is simple and minimally invasive, and most surgeries can be performed in < 30 minutes. It is not necessary to limit weight-bearing with external coaptation or postoperative bandages.
This method determines the position and angle of pin insertion intraoperatively using dynamic evaluation. However, the optimal margin was narrow, and further studies are required to determine the correct pin orientation. In one of the cases in which reluxation occurred postoperatively, the pin insertion was reevaluated by revision surgery and the original pin was replaced to increase the antiluxation effect. In 1 of the 3 cases in which postoperative lameness persisted, the pin insertion was repositioned by 2 revision surgeries to decrease the antiluxation effect, resulting in satisfactory results over a long period of time. While there was no reluxation in 1 case, the recurrence of lameness was confirmed by long-term follow-up observation. Possible explanations for this include excessive interference of the pin with the supraspinatus muscle or the progression of preexisting osteoarthritis. Excessive pin interference with the supraspinatus muscle tendon or the glenoid may lead to discomfort and lameness. The placement of an intra-articular pin may cause synovitis. However, in the cases where the pin was appropriately inserted, these complications did not occur and no clinical issues arose during long-term follow-up.
In this study, cases with significant damage to the skeletal or supporting structures of the shoulder joint due to trauma and cases with severe congenital morphological abnormalities were considered unsuitable for this surgical procedure. Further studies are necessary to determine how the morphological abnormalities of the shoulder joint, instability due to damage to the supporting structures, and the degree of degenerative joint disease affect the feasibility of treatment. Although we did not meticulously evaluate the damage to the medial glenohumeral ligament, subscapularis, and joint capsule, prevention of reluxation and long-term preservation of walking function were confirmed postoperatively. It is possible that the pin in the humerus effectively functioned as an active stabilizer, improving long-term walking conditions without the need to treat damaged passive stabilizers. Further investigations are required to determine whether this surgical procedure prevents the progression of mild or moderate shoulder instability.
As this was a retrospective study, there were no established criteria for the application of surgery. Additionally, several orthopedic evaluations such as shoulder joint muscle atrophy, crepitus, and measurements of range of motion were lacking, especially in the dogs for which the last follow-up evaluation was conducted through a telephone interview with the owner. While LMS was used to evaluate walking function before and after surgery, the use of force plates and kinematic studies are desirable for a more detailed evaluation.
Placement of an antiluxation pin lateral to the supraspinatus muscle tendon prevented the recurrence of and maintained a good long-term function in most toy-breed dogs with MSL. This novel technique is minimally invasive and technically simple and is an effective surgical treatment for toy-breed dogs with MSL.
Supplementary Materials
Supplementary materials are posted online at the journal website: avmajournals.avma.org.
Acknowledgments
None reported.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
The authors have nothing to disclose.
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