Medial Shoulder Instability
Medial shoulder instability or MSI is a common cause of shoulder pain and lameness. We know from the name that this is a condition that affects the medial or inside of the shoulder, and involves failure in the stabilizing tissues. So this is essentially the equivalent of a rotator cuff injury in humans.
My Full Length Medial Shoulder Instability Video if you prefer to watch vs read.
ANATOMY
The shoulder joint in the dog is a ball and socket joint. The head of the humerus (the arm bone) is the 'ball', and the glenoid cavity/cup on the scapula is the 'socket'. The head of the humerus is larger than the glenoid cup. This makes it what we call an incongruent joint. This large ball (humeral head), shallow socket, allows for more mobility, the greatest range of motion of any joint in the dog's body, but at the cost of bony stability. Every joint must have some balance between mobility and stability. The hip will favor stability and shoulder mobility. So this is an inherently mobile but unstable joint.
Of course the bones are only one part of the joint. Today we will focus just on the medial side of the joint. The shoulder is held together by passive and dynamic stabilizers. Damage to all, or some, of these structures could result in Medial Shoulder Instability. That is why you may sometimes hear Medial Shoulder Instability referred to as a "waste basket" diagnosis because it isn't specific and refers to the symptom rather than the exact injury.
There are two main passive stabilizers.
The first is the Medial Glenohumeral Ligament, depicted in either a 'Y' or 'trident' shape, it goes from the distal scapula to the lesser tubercle on the humerus. There is a cranial arm or branch and a caudal arm, as they move distally they join together to insert into the humerus. This is a relatively weak ligament that is commonly affected in Medial Shoulder Instability. Even in healthy dogs this ligament relies heavily on support from other stabilizers.
The Medial Glenohumeral Ligament is actually inside and a part of the second passive stabilizer; the joint capsule.
The joint capsule surrounds the joint providing stability and protection. Starting from the outside of the glenoid cavity it goes down and attaches just below the articular part of the humeral head; there it blends into the periosteum on the neck of the humerus and surrounds the biceps brachii tendon here in the intertubercular groove. Inside the joint capsule synovial fluid fills the joint space to reduce friction and keep the tissues healthy.
A balance between synovial fluid production and absorption must be maintained to achieve the correct interstitial fluid pressure IFP. Having the correct IFP keeps the joint healthy by distributing forces evening across the joint. Not enough synovial fluid can lead to weight not being evenly dispersed across the joint, too much would lead to excessive pressure and pain in the joint. Think of someone with a swollen knee that needs to go to the doctor to get some synovial fluid drained out to alleviate the pressure.
The main dynamic stabilizer is the Subscapularis Tendon. The Subscapularis muscle originates from, and occupies the subscapular fossa, slightly overlapping these boundaries both cranially and caudally. It is a relatively flat muscle that is divided into a few sections by tendinous bands. It crosses over the shoulder joint and Medial Glenohumeral Ligament inserting into the lesser tubercle by a short thick tendon, uniting here with the joint capsule. This serves a few functions. First it allows the joint capsule and Subscapularis Tendon to insert into the same area. Tendons generally like to attach right after the joint on which they exert their main action. Having their insertion point near the joint increases the speed at which they can move the joint. So both structures fusing together allows them to share preferential space on the bone.
Highly mobile joints need a certain degree of laxity in the joint capsule to allow the joint to move through its full range of motion. This increased laxity does increase the risk of the joint capsule getting pinched within the joint. Merging of the tendon and the joint capsule decreases the risk of capsular entrapment. And it easily allows the muscle to compress and tense the capsule. Though a tendon can pass over a joint and press on the joint capsule without being fused to it.
The Subscapularis Tendon functions like a medial collateral ligament. Activation of this muscle compresses the joint capsule and creates dynamic stability. In addition to its action as a stabilizer it is also a shoulder extender, though it can also play a role contributing to maintaining shoulder flexion. It also aids in adduction of the shoulder and internal rotation. It is innervated by the Subscapular nerve and axillary nerve and bordered by the Supraspinatus cranially and the Teres Major caudally.
Some sources say that injury to the Subscapularis Tendon is the most common component of Medial Shoulder Instability. Others will say the Medial Glenohumeral Ligament. Both are very important medial stabilizers that can be injured resulting in Medial Shoulder Instability. Severe Medial Shoulder Instability will generally involve injury to both. It seems rational that a chronic injury of one, if left untreated long enough will result in injury to the other as it will be overloaded trying to compensate.
Cutting the Subscapularis Tendon in cadavers did result in Medial Shoulder Instability. Cutting the cranial band of the Medial Glenohumeral Ligament did not result in instability right away. The inflammatory response weeks later was what seemed to result in Medial Shoulder Instability.
HOW Medial Shoulder Instability HAPPENS
There are two ways that Medial Shoulder Instability can develop. A chronic or repetitive injury is when the dog has overloaded the affected tissue via small repeated traumas until one day the Medial Glenohumeral Ligament or Subscapularis Tendon suddenly fails during the course of normal daily activities. Jumping off the couch, agility exercises like weave poles, a minor slip on the floor, etc.
A traumatic injury is what it sounds like, a sudden injury resulting from a significant overload of the tissues. A bad slip/fall, accident etc.
Muscle atrophy or damage to some of the other surrounding muscles can put more load on the Subscapularis Tendon and Medial Glenohumeral Ligament resulting in injury. Congenital abnormalities in either scapula or humerus could also lead to Medial Shoulder Instability. If the glenoid cup lacks concavity and isn't very much of a cup, or if the proximal humerus is misshapen then that dog will be at a much higher risk of developing Medial Shoulder Instability. The muscles and ligaments will have to work much harder to compensate for the boney defects. Smaller breed dogs are more prone to these birth defects much like we see with patellar luxation. Their muscles will have to work harder to compensate for the decreased bony stability.
Regardless of whether the injury is chronic or traumatic the injury is due to excessive abduction. The arm moving too far away from the body under too much strain. The Medial Glenohumeral Ligament and Subscapularis Tendon on the inside of the shoulder joint both limit excessive abduction. When these structures are put under heavy load whether from a slip, weave poles or other cause, sudden or progressive failure can occur.
Medial Shoulder Instability most commonly affects agility and working dogs from 4-5 years old. Dogs who run hard and put these relatively weak structures under heavy and consistent load.
Interestingly poodles are also over represented in Medial Shoulder Instability injuries. They seem to have some genetic propensity to this injury that affects pure breed and 50% poodle crosses as well. It isn't clear the exact reason. The two main variables in joint instability are joint anatomy and soft tissue laxity so it would be one of these or a combination of the two. But whatever the cause, the numbers in one Australian study were very decisive. (Woolley et al, 2023) Poodles were over represented 13 times in Medial Shoulder Instability. So Medial Shoulder Instability should definitely be considered in any poodle showing forelimb lameness.
In other breeds about 4-5% of forelimb lameness is Medial Shoulder Instability. It is the most common type of shoulder instability, about 60-80% of shoulder instability is Medial Shoulder Instability. Most of the remainder are lateral. Cranial and caudal instability are very rare.
Medial Shoulder Instability GRADING
There are four grades of Medial Shoulder Instability
Grade 1: Mild Medial Shoulder Instability
Inflammation or laxity but no fraying or tears to Subscapularis Tendon or Medial Glenohumeral Ligament
Grade 2: Moderate Medial Shoulder Instability
This is the most common type of Medial Shoulder Instability. This could involve fraying/partial tear but not a full tear to the Medial Glenohumeral Ligament and/or Subscapularis tendinopathy. More than half the time there is also concurrent supraspinatus tendinopathy, says one source.
Grade 3: Severe Medial Shoulder Instability
This is when there is a complete tear to the Subscapularis Tendon or Medial Glenohumeral Ligament, or both. This is much less common but obviously a very severe injury that will usually be the result of trauma.
Grade 4: Luxation
Complete tear of Subscapularis Tendon and Medial Glenohumeral Ligament resulting in complete displacement of the humeral head
SYMPTOMS
If the injury is mild, symptoms could be subtle, such as a shortened stride, reduced shoulder extension, atrophy, muscle spasm, pain on abduction. If the injury is very serious it could lead to full lameness on the affected limb.
Lameness if present will typically be worse after exercise.
Noticing these symptoms early and pursuing a correct diagnosis and treatment will stop this condition from progressively worsening leading to further instability and the development and progression of OA.
ASSESSMENT
Even though it is called medial shoulder 'instability' it does not always result in detectable instability. Some people have suggested the term medial shoulder syndrome as a more accurate description but Medial Shoulder Instability is still the predominant term. Assessment for Medial Shoulder Instability will generally involve hands-on palpation assessment and imaging.
The most commonly performed test is an Abduction test. There are two versions of this test and ideally both will be performed and compared for the best results. First the dog can be awake and standing. One forelimb will be slightly extended forward while straight. The arm will then be put into abduction with the practitioner slowly moving the arms away from the dog's body. This test can also be done while the dog is laying on its side in lateral recumbency. Using a goniometer across the spine of the scapula and the humerus the maximum abduction angle can be measured. The shoulder joint will also generally be moved in circumduction with internal and external rotation to check for any subluxation. This is when the head of the humerus is moving too far away from the glenoid cup, indicating severe injury. The primary goal of the awake version of the test is to evaluate pain.
Abduction Test Video (not mine but good quality)
With the dog sedated this test can be repeated on their side to get a more accurate abduction angle measurement. When the dog is sedated the active stabilizers like the Subscapularis Tendon won't fire/or won't fire as much. This is important because if the injury is to the Medial Glenohumeral Ligament but not the Subscapularis Tendon it may be able to tense up and compensate for the dog when awake, leading to a 'normal' abduction test in an injured dog.
The average healthy dog will have an abduction angle of 30 degrees. The average dog with Medial Shoulder Instability will have an abduction angle of 50-55 degrees. However this test is not definitive. Some dogs with Medial Shoulder Instability will not have an increased abduction angle. There will be injury to the Medial Glenohumeral Ligament and/or Subscapularis Tendon but not so severe to lead to laxity in this test. Muscle overactivation and guarding can limit the abduction angle and results of this test. Not all dogs with an abduction angle above 50 will have Medial Shoulder Instability either. Some dogs have been reported to have abduction angles up to 75 degrees in a healthy shoulder! Normal abduction angle can also vary within breeds. 45 degrees is the normal healthy average in sled dogs for example. So why do this test at all then? While it isn't conclusive, an abduction angle around 30 degrees still makes Medial Shoulder Instability less likely and above 50 degrees means it is more likely. So it can still inform which tests to prioritize next. Also both the affected arm and healthy arm should be tested. If there is a significant difference in abduction angles between the arms that would indicate a higher likelihood that Medial Shoulder Instability may be a contributing factor. An abduction angle difference of 15 degrees or more is considered strongly suggestive of Medial Shoulder Instability. Even if both angles are within the normal range. If there is pain on abduction that is also a strong indicator of potential Medial Shoulder Instability.
If there is atrophy of the shoulder muscles. Pain on palpation. Notable spasms in the surrounding muscles. These could also indicate Medial Shoulder Instability. Also while range of motion will generally be increased in abduction, range of motion in other directions, particularly extension will often be limited. Supraspinatus tendinopathy should also be tested by flexing the dog's shoulder and palpating the tendon of insertion.
MRI, Ultrasound, or arthroscopy can be used to look at the tendons and ligaments for fraying or tearing. Because MRI is very expensive and requires full sedation it is not commonly used. Ultrasound is very good for looking at tendons, muscles and ligaments. However it can be harder to see the medial/inside of the shoulder, especially if the dog is overweight. It can often be done with mild sedation, or sometimes even without and is generally available at most clinics. Ultrasound can also be done dynamically. Meaning that you can move the shoulder while performing ultrasound to watch the tissues in real time. Shoulder arthroscopy is generally thought of as the gold standard though and needle arthroscopy can be done under sedation to look at muscles, tendons and ligaments.
We must also remember to rule out other issues such as elbow pathology, shoulder ocd etc. If a dog has been lame on a front limb for a long time they could have disuse atrophy that results in weakness of the stabilizing muscles and leads to laxity in the shoulder. This could result in an abnormal abduction angle. However this laxity may not be Medial Shoulder Instability or it may be mild Medial Shoulder Instability but secondary to an earlier primary injury. This is why other imaging such as X-ray and CT scans may be used. Not to diagnose Medial Shoulder Instability but to look for other injuries that could be causing or contributing to the Medial Shoulder Instability related symptoms. You could also of course have multiple independent injuries as well. X-ray could be used to diagnose a severe Grade 4 Medial Shoulder Instability because the luxation would show up with the bones out of place.
TREATMENT
If the injury is severe. The articular cartilage is wearing. The head of the humerus is luxating. Then surgery is often recommended.
Otherwise conservative management is usually attempted first. Rest, rehab, pain management, shockwave, laser, platelet rich plasma/platelet lysate, stem cells, stretching (not abduction), massage, NMES for atrophy etc. Note that the teres major will often be very tight to try to compensate, and massage and laser will likely be beneficial here.
The dog can then be reassessed after say 3 months to see what progress if any has been made. Remeasuring and comparing abduction angles can help to monitor progress.
Radiofrequency Induced Thermal Capsulorrhaphy (RITC) is an intermediate option that may be used to treat Medial Shoulder Instability grades 1 and 2. It is used to heat up the tissues. The heat causes the collagen to contract and the ligament and/or joint capsule to shrink and tighten, treating the laxity. In the past multiple passes were done but they found that this overheated the tissues resulting in damage. One pass with the tool is now the standard to adequately shrink the targeted tissues without damage. This won't work on grades 3 and 4 as those involve full tears and shrinking 2 ends of a cut rope won't reattach them. But of grades 1 and 2, laxity or partial tears, outcomes are pretty good overall but the full healing/benefits can take 6 months. This is an arthroscopic procedure and won't require a large incision.
For cases requiring full surgery the Canine Tightrope System is often used. Holes are drilled in the bones and then ropes are pulled through to tighten the joint down. These ropes are placed superficial to the joint capsule. Biceps and supraspinatus tendon transposition are also used. In general tendon transpositions tend to alter shoulder biomechanics. This makes sense as tendons are now inserting into different locations and performing functions that deviate from their natural movements. This will result in more degenerative changes over time. Either way the goal is the same to reconstruct medial support for the glenohumeral joint.
Woolley et al, (2023), Medial Shoulder Instability: Prevalence and Treatment Outcomes in 17 Poodles and 31 Dogs of Other Breeds