Elbow Instability: Understanding PLRI and VPMRI
- Lucas Myerson
- Mar 28
- 9 min read
A Practical Framework for Understanding Posterolateral Rotatory Instability (PLRI) and Varus Posteromedial Rotatory Instability (VPMRI) — from stabilizer anatomy and injury mechanism to diagnosis and surgical decision-making.
Elbow instability is one of the more under-appreciated problems in upper extremity surgery. It tends to present atypically, get misdiagnosed, and when treated incorrectly, leads to persistent pain and mechanical symptoms that can be difficult to salvage. PLRI and VPMRI are two distinct patterns that share the elbow joint but differ substantially in mechanism, pathoanatomy, and surgical management. Understanding both — and the anatomy that underlies them — is essential to treating either one well.
The Architecture of Elbow Instability
Before discussing what goes wrong, it helps to be clear on how the elbow stays stable in the first place. The short answer is that it relies on three overlapping systems organized in a functional hierarchy.
The Rule of Three: Bony Stabilizers
The elbow is a three-joint articulation — the ulnohumeral, radiocapitellar, and proximal radioulnar joints — and it has three primary bony stabilizers: the coronoid, the radial head, and the olecranon. Each of these contributes to stability in a complementary way, and the loss of any one of them has implications for how the remaining structures respond to load.
Bony geometry sits at the base of the stability hierarchy, followed by the ligamentous restraints, and finally the dynamic muscle stabilizers. The muscles act as a moat — providing the last line of defense when the structural constraints have been compromised.
Static Stabilizers: The Ligamentous Complexes
The medial side of the elbow is stabilized by the ulnar collateral ligament (UCL) complex, which consists of the anterior medial collateral ligament (A-MCL), the posterior medial collateral ligament (P-MCL), and the transverse ligament. The A-MCL is the primary medial restraint and the workhorse of valgus stability.
On the lateral side, the lateral collateral ligament (LCL) complex includes the radial collateral ligament (RCL), the annular ligament, and critically, the lateral ulnar collateral ligament (LUCL). The LUCL is the essential restraint against posterolateral rotatory subluxation. It runs from the lateral epicondyle to the supinator crest of the ulna, and its disruption is the defining lesion of PLRI.
O'Driscoll's Fortress Concept
O'Driscoll's framework for elbow stability is intuitive and worth committing to memory. Think of it as a fortress with concentric walls.
The primary constraints — the UCL complex on the medial side, the LUCL on the lateral side and the ulnohumeral articulation — form the first wall. The secondary constraints — the common extensor, flexor-pronator tendons and radiocapitellar articulation — form the second wall. The anterior and posterior capsule provide the moat. Stability is maintained as long as enough of these layers are intact. The clinical implications follow directly: more layers disrupted means less residual stability and a more demanding surgical problem.
Mechanism of Injury: Circle of Horii
The classic elbow dislocation mechanism involves a combination of axial compression, valgus stress, and forearm supination — typically from a fall onto an outstretched hand. This creates a predictable sequence of ligamentous disruption, described by Horii and colleagues as a circle of sequential failure that proceeds from lateral to medial.
Stage 1 begins with disruption of the LUCL, producing posterolateral subluxation with spontaneous reduction (PLRI). As the circle progresses anteriorly through the anterior capsule and posterior capsule, Stage 2 produces a perched dislocation where the coronoid sits on the trochlea. Full posterior dislocation — Stage 3 — involves complete disruption of the lateral and medial stabilizers, and can be stable (3a: anterior MCL intact) or unstable (3b: full MCL disruption) after reduction. Stage 3c represents the most severe pattern, with the elbow grossly unstable in all planes after reduction due to complete circumferential disruption.
VPMRI follows an entirely different mechanism and is a distinct injury pattern from PLRI. Rather than supination and valgus, VPMRI results from varus stress combined with axial compression and internal rotation of the forearm — often from a fall in which the arm is internally rotated at the shoulder. This produces a shear force across the anteromedial facet of the coronoid rather than the posterolateral soft tissue complex, and VPMRI should therefore be thought of as a fracture-instability pattern rather than a pure ligamentous injury.
Patterns of Instability: Pathoanatomy
PLRI: The Most Common Form of Chronic Elbow Instability
PLRI is the most common pattern of chronic elbow instability in adults. The essential lesion is disruption of the LUCL at its humeral origin — the lateral epicondyle — though midsubstance tears and distal avulsions also occur. The injury can be acute following a traumatic dislocation, chronic following incomplete healing, iatrogenic following lateral elbow surgery (particularly for lateral epicondylitis, where the common extensor origin and LUCL are at risk), or related to deformity as in cubitus varus.
The functional consequence is recurrent subluxation of the ulnohumeral joint in a posterolateral direction. The radial head pivots posterolaterally relative to the capitellum, producing the characteristic symptom cluster of lateral elbow pain, clicking, clunking, or a sense of giving way with specific provocative activities — typically elbow extension from a flexed, supinated position under load.
VPMRI: A Fracture-Instability Pattern
VPMRI is defined by fracture of the anteromedial facet of the coronoid process combined with disruption of the lateral collateral ligament complex. The anteromedial coronoid fracture is classified according to O'Driscoll's system into three subtypes based on location: subtype 1 involves the tip of the coronoid, subtype 2 extends to the anteromedial rim, and subtype 3 involves the sublime tubercle — the medial attachment of the A-MCL. This distinction matters surgically because subtype 3 fractures require MCL repair in addition to coronoid fixation.
The instability pattern in VPMRI is a posteromedial rotatory subluxation — the exact geometric opposite of PLRI. The ulna rotates internally and the olecranon subluxates posteromedially relative to the distal humerus. On radiographs, this produces the characteristic drop sign: widening of the ulnohumeral joint medially on a lateral view, reflecting loss of the normal congruent relationship between the trochlea and the coronoid.
Presentation and Diagnosis
PLRI — History and Examination
Patients with PLRI often have an antecedent history of elbow dislocation or surgery, though the connection to prior trauma is not always recognized. The chief complaint is lateral elbow pain with mechanical symptoms — clicking, giving way, or apprehension with use. Activities involving pushing up from a chair, doing push-ups, or extending the elbow against resistance with the forearm supinated are particularly provocative.
The two most important physical examination tests for PLRI are the lateral pivot-shift test (described by O'Driscoll in 1991) and the chair push-up test. The lateral pivot-shift test is performed supine with the shoulder flexed overhead: the examiner applies a valgus moment, supination, and axial compression while extending the elbow from a flexed position. A positive test reproduces apprehension or visible subluxation of the radiocapitellar joint, with a clunk on reduction. The test is often easier to perform under anesthesia; awake patients may not fully relax. The chair push-up test is a functional analog: the patient rises from a chair by pushing up with forearms supinated, reproducing symptoms with elbow extension.
MRI and MR arthrogram can identify LUCL disruption at the lateral epicondyle, though sensitivity varies. The imaging finding is signal abnormality at the LUCL footprint, best seen on coronal sequences. MR arthrogram improves sensitivity by introducing contrast that extravasates through a torn ligament.
VPMRI — History and Examination
VPMRI typically presents acutely following a fall. Patients have medial elbow pain and swelling, and on examination the elbow may have restricted motion due to pain and effusion. The drop sign on lateral radiograph — posteromedial subluxation of the ulnohumeral joint — is the key imaging finding and should trigger 3D CT for fracture characterization.
The gravity varus stress test is the examination maneuver for VPMRI. With the patient supine and the shoulder in 90 degrees of abduction and the elbow at 90 degrees of flexion, gravity applies a varus moment to the elbow. Opening of the ulnohumeral joint medially, or pain reproduction, constitutes a positive test. The test exploits the same varus vector responsible for the injury mechanism.
3D CT is essential for VPMRI. Plain radiographs frequently underestimate the size of anteromedial coronoid fractures, and the subtype cannot be reliably determined without three-dimensional reconstruction. The distinction between subtype 1, 2, and 3 fractures directly determines whether MCL repair needs to be added to coronoid fixation and LCL repair, so CT is not optional in this injury pattern.
Treatment Strategies
Non-Operative Management
Non-operative management has a role in selected patients with PLRI, specifically those who are low-demand, older, or who have mild symptoms amenable to activity modification. The mechanical rationale for non-operative treatment involves rehabilitation of the dynamic stabilizers — particularly the pronator muscle group — to compensate for LUCL deficiency. Positioning the forearm in pronation tightens the anterior interosseous structures and reduces the tendency toward posterolateral subluxation. A hinged elbow brace in pronation can offload the LUCL while rehab proceeds.
Non-operative management for VPMRI with instability is generally contraindicated. The anteromedial coronoid fracture creates a structural deficit that dynamic muscle rehabilitation cannot compensate for. If the elbow is unstable — defined by the drop sign on imaging or clinical testing — operative fixation is the correct treatment. Delay in surgery for VPMRI risks articular incongruence, cartilage loss, and progressive posteromedial arthritis.
Surgical Treatment for PLRI
Surgical treatment of PLRI depends on whether the injury is acute or chronic. In acute PLRI following a recognized dislocation, primary repair of the LUCL to the lateral epicondyle is appropriate. The ligament tears from its humeral origin in the majority of cases, and repair is performed using suture anchors or bone tunnels to restore isometric attachment at the lateral epicondyle.
Suture tape augmentation — where a high-strength suture tape is placed alongside the primary repair to protect it from cyclic loading during healing — has gained traction as an adjunct to acute LUCL repair. The concept mirrors its use in other ligament repairs: the tape provides immediate load-sharing, allowing the repaired tissue to heal without being cyclically overloaded in the early postoperative period.
For chronic PLRI, where the native LUCL has scarred or attenuated beyond the capacity for meaningful primary repair, ligament reconstruction is indicated. The technique involves grafting — typically with palmaris longus, gracilis, or a strip of triceps tendon — through bone tunnels in the lateral epicondyle and supinator crest of the ulna. The graft is passed in a figure-eight or docking configuration to recreate LUCL function. Results from the Mayo Clinic series by Sanchez-Sotelo, Morrey, and O'Driscoll showed satisfactory outcomes in the majority of patients with appropriately selected reconstruction, though recurrence and residual instability remain the most common reasons for failure.
Key Surgical Principles for PLRI
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Surgical Treatment for VPMRI
The operative principles for VPMRI are well-established: fix the coronoid, repair the LCL, and repair the MCL if the sublime tubercle is involved (O'Driscoll subtype 3). All three elements of the injury must be addressed to restore stability. Treating only the coronoid and neglecting the LCL repair reliably results in recurrent instability. Similarly, treating only the soft tissue injury without addressing the coronoid fracture leaves the medial column deficient and allows progressive articular subsidence.
Approach to the anteromedial coronoid depends on fracture size and accessibility. Small fragments may be fixable through a medial approach using a plate or suture fixation. Larger fragments are approached through a medial skin incision with elevation of the flexor-pronator mass, developing a plane between the brachialis anteriorly and the pronator mass to expose the anteromedial coronoid. Fixation is with lag screws, plate, or suture lasso depending on fragment size and bone quality.
The lateral incision for LCL repair is performed through the Kocher or extended Kocher interval, with the LUCL repaired directly to the lateral epicondyle. If the LCL tissue is avulsed but repairable, suture anchor repair is performed. If attenuated, reconstruction with a graft may be needed, though acute LCL avulsions in the context of VPMRI are usually amenable to primary repair.
Key Surgical Principles for VPMRI
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A Decision Framework: PLRI vs VPMRI
Variable | PLRI | VPMRI |
Primary lesion | LUCL disruption (humeral origin) | Anteromedial coronoid fracture + LCL tear |
Mechanism | Valgus + supination + axial load (FOOSH) | Varus + internal rotation + axial load |
Key finding on exam | Lateral pivot-shift test; chair push-up test | Gravity varus stress test; drop sign on XR |
Imaging workup | MRI or MR arthrogram for LUCL | 3D CT essential for coronoid classification |
Non-op role | Selected low-demand patients; pronation rehab | Generally contraindicated if unstable |
Surgical goal | Restore LUCL: repair (acute) or reconstruct (chronic) | Fix coronoid + repair LCL ± MCL repair |
Critical pitfall | Iatrogenic LUCL injury from lateral elbow surgery | Missing the instability behind an 'isolated' coronoid Fx |
The Bottom Line
PLRI and VPMRI are fundamentally different injuries that require different approaches to imaging, examination, and surgery. PLRI is a ligamentous problem centered on the LUCL and is the most common form of chronic elbow instability. It can be managed non-operatively in selected patients, but most active patients with symptomatic PLRI will eventually require surgical reconstruction. VPMRI is a fracture-instability pattern involving the anteromedial coronoid that should not be treated conservatively if instability is present — the structural deficit is osseous, not just soft tissue, and fixation of the coronoid combined with LCL repair is the standard of care.
In both cases, accurate diagnosis — including appropriate advanced imaging — is the critical first step. MR arthrogram for suspected PLRI, and 3D CT for any coronoid fracture that might represent VPMRI, are not optional studies. They are the foundation on which sound surgical decision-making rests.
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About the Author
C. Lucas Myerson, MD – Orthopedic surgeon specializing in shoulder and elbow surgery.
Disclaimer
This article is for educational purposes only. It is not a substitute for medical advice. Always talk to your doctor before starting or changing treatment.