Pisiform Dislocation Radiology: X-Ray, CT, and MRI Findings

Pisiform dislocation is a rare carpal injury that is frequently missed on initial emergency imaging, largely because standard anteroposterior and lateral wrist radiographs often fail to show it clearly. CT scanning is the most reliable way to confirm the diagnosis and characterize the direction of displacement. Because the pisiform sits directly over Guyon’s canal, a missed dislocation can compress the ulnar nerve and lead to lasting neurological problems, making prompt radiological recognition more than an academic exercise.

Why the Pisiform Is Vulnerable and Hard to See

The pisiform is the smallest of the carpal bones and the only one classified as a sesamoid, meaning it develops within a tendon rather than forming as part of the main bony arch of the wrist. It sits embedded in the flexor carpi ulnaris (FCU) tendon on the palmar-ulnar side of the wrist, articulating with just one other bone: the triquetrum. That single articulation, known as the pisotriquetral joint, is stabilized by a network of ligaments and soft tissue attachments. Biomechanical testing on cadaver wrists has shown that the pisotriquetral joint’s primary stabilizers are the pisometacarpal, pisohamate, and ulnar pisotriquetral ligaments, which resist proximal, ulnar, and radial displacement forces respectively.1PubMed. The pisotriquetral joint: anatomic, biomechanical, and radiographic analysis The soft tissues surrounding the joint are strongest on the proximal and distal sides but weakest on the medial aspect, which helps explain why the pisiform can displace when those restraints are overwhelmed.2PubMed. Ligamentous and tendinous support of the pisiform, anatomic and biomechanical study

On a standard posteroanterior wrist radiograph, the pisiform overlaps the triquetrum and other carpal bones, making subtle displacement almost invisible. Even on a lateral view, the pisiform’s small size and its overlap with the hook of the hamate and the palmar soft tissues conspire to hide it. This superimposition is the core radiology problem: you are looking for a pea-sized bone that has shifted a few millimeters in a field crowded with overlapping structures.

How Pisiform Dislocations Happen

Two main injury mechanisms appear repeatedly in the literature. The first is a sudden forceful dorsiflexion of the wrist, which stretches the FCU tendon sharply and levers the pisiform away from the triquetrum. The second is a direct, powerful contraction of the FCU itself, tearing the pisohamate and pisotriquetral ligaments and pulling the pisiform out of position.3Journal of Orthopaedic Case Reports. Isolated Compound Dislocation of the Pisiform Bone with Associated Neurovascular Injury Following an Assault: A Rare Injury Pattern Falls onto an outstretched hand, direct blows to the palm, and even violent seizure episodes have all been reported as triggers. One published case described a 31-year-old woman whose pisiform dislocated during a seizure-related fall, with the injury going unrecognized for three weeks before CT confirmed it.4Trauma Case Reports. Isolated pisiform dislocation after a seizure episode

Because the mechanisms overlap with far more common injuries like wrist sprains, distal radius fractures, and simple contusions, clinicians often do not suspect the pisiform specifically. The patient presents with pain, swelling, and tenderness on the ulnar side of the wrist, which is a nonspecific pattern shared with many other conditions. That vagueness at the bedside places an even heavier burden on imaging to catch what the physical exam cannot.

Conventional Radiography and Its Limits

Standard AP and lateral wrist radiographs are the first imaging study ordered for virtually every acute wrist injury, but they have well-documented blind spots for the pisiform. In one published case, emergency physicians reviewed the initial radiographs and failed to identify the pisiform dislocation entirely; the diagnosis only came after a CT scan was obtained.5Journal of Surgical Case Reports. Isolated acute pisiform dislocation presenting atypically as wrist sprain in the emergency department This is not an isolated incident. Pisiform dislocation is consistently described in the literature as an injury that is easily missed in an acute emergency presentation, and a lack of familiarity with its characteristic imaging appearance is a major contributor to delayed diagnosis.6PubMed Central. An uncommon case of traumatic pisiform dislocation with triquetral fracture

Oblique radiographs, particularly the carpal tunnel or supinated oblique view, can improve visualization of the pisotriquetral articulation by reducing overlap. Comparison with the contralateral wrist on these views is often helpful, because a subtle asymmetry in the pisiform’s position relative to the triquetrum may be the only clue.7Journal of Surgical Case Reports. Isolated acute pisiform dislocation presenting atypically as wrist sprain in the emergency department Still, even with optimized views, plain films remain a screening tool for this injury rather than a definitive one. If clinical suspicion persists after normal-appearing radiographs, the next step is CT.

CT as the Definitive Diagnostic Tool

CT scanning resolves the superimposition problem that undermines plain radiographs. Thin axial slices through the wrist show the pisiform and triquetrum in cross-section, making even a few millimeters of displacement obvious. Sagittal reconstructions are particularly valuable because they display the pisotriquetral joint in profile, revealing whether the pisiform has migrated volarly (toward the palm), dorsally, or in another direction. In one published case, sagittal CT sections demonstrated volar migration of the pisiform with a markedly widened pisotriquetral joint space and complete loss of the normal congruent articular relationship, confirming a volar dislocation pattern.8PubMed Central. Concomitant Distal Radius Fracture, Scaphoid Fracture, and Pisiform Dislocation with Ulnar Nerve Neuropraxia: A Case Report with Biomechanical Analysis

Beyond simply confirming whether the pisiform is in or out of joint, CT provides information that directly affects treatment planning. The direction of displacement, whether volar, dorsal, radial, or ulnar, dictates the corrective force needed during closed reduction. Researchers have argued that systematic description of displacement direction on CT should be standard in all reported cases to build a classification framework and guide treatment decisions.9Journal of Orthopaedic Case Reports. Concomitant Distal Radius Fracture, Scaphoid Fracture, and Pisiform Dislocation with Ulnar Nerve Neuropraxia: A Case Report with Biomechanical Analysis This is a practical point for anyone interpreting the scan: do not simply state that the pisiform is dislocated. Describe which way it went.

What to Look for on CT

A radiologist or clinician reviewing a wrist CT for suspected pisiform dislocation should check for several specific findings:

These findings together give the treating surgeon a detailed picture: the severity of displacement, the integrity of surrounding bones, and the risk of neurovascular compromise.

The Role of MRI

MRI does not replace CT for confirming the bony dislocation itself, but it adds a layer of information CT cannot provide. Where CT excels at showing bone position and fracture lines, MRI reveals the soft tissue damage that goes along with the dislocation: torn pisotriquetral and pisohamate ligaments, FCU tendon injury, cartilage damage to the pisotriquetral joint surfaces, and edema in the surrounding structures. In cases where both radiographs and MRI were obtained, the combination revealed injuries like pisiform dislocation with an associated triquetral fracture that might have been incompletely characterized by either modality alone.12PubMed Central. An uncommon case of traumatic pisiform dislocation with triquetral fracture

In practice, MRI tends to be ordered when the clinical picture is subacute or chronic rather than in the acute emergency setting. A patient who presents weeks after an injury with persistent ulnar-sided wrist pain and whose initial radiographs were read as normal might undergo MRI as the next diagnostic step. The ligamentous disruption and bone marrow edema visible on MRI can then point toward the pisotriquetral joint as the source of symptoms and prompt a targeted CT for definitive bony assessment.

Ulnar Nerve Risk and Why It Matters Radiologically

The pisiform forms part of the floor of Guyon’s canal, a narrow fibro-osseous tunnel on the ulnar side of the wrist through which the ulnar nerve and artery pass. The average distance between the pisiform and the ulnar nerve is less than a centimeter, measured at roughly 0.89 cm in anatomic studies.13PubMed Central. An Unusual Presentation of Isolated Pisiform Dislocation: A Case Report of Dorsal and Ulnar Displacement When the pisiform dislocates, that tiny margin can vanish. The displaced bone may directly contuse or compress the ulnar nerve, causing numbness and tingling in the ring and small fingers, weakness in the hand’s intrinsic muscles, or both.

For the radiologist, this proximity means that any CT or MRI report describing a pisiform dislocation should specifically comment on the relationship of the displaced bone to Guyon’s canal. If the pisiform has shifted in a direction that narrows or obliterates the canal, that information should be flagged prominently, because it may lead the surgeon to perform an ulnar nerve release at the time of definitive surgery rather than waiting to see whether symptoms develop postoperatively.14PubMed Central. An Unusual Presentation of Isolated Pisiform Dislocation: A Case Report of Dorsal and Ulnar Displacement Compound dislocations, where the pisiform breaks through the skin, carry an even higher risk of neurovascular injury.15Journal of Orthopaedic Case Reports. Isolated Compound Dislocation of the Pisiform Bone with Associated Neurovascular Injury Following an Assault: A Rare Injury Pattern

Pisiform Dislocation in Children

Pediatric pisiform dislocation presents a unique radiological challenge because the pisiform is one of the last carpal bones to ossify. It typically does not appear on radiographs until around age nine to twelve, and even once ossified it may be small and irregularly shaped for several more years. In young children, the pisiform may be entirely cartilaginous and invisible on plain films and CT alike, making ultrasound or MRI necessary if the injury is suspected clinically.

Published pediatric cases are extremely rare. One report described a fracture-dislocation of the pisiform in an 11-year-old boy who also had a greenstick radial fracture, treated with open reduction and internal fixation.16PubMed Central. Pisiform Bone Dislocation in a Pediatric Patient: What is the Best Treatment? Case Report and Review of the Literature The literature on pediatric management is thin enough that there is no consensus on whether closed reduction, open reduction with fixation, or excision is the best approach. Recurrent dislocation after both closed and open reduction techniques has been reported.17PubMed Central. Pisiform Bone Dislocation in a Pediatric Patient: What is the Best Treatment? Case Report and Review of the Literature The radiologist reading a pediatric wrist study should keep the pisiform on their mental checklist even when it barely registers as a visible structure.

Associated Injuries and Combined Patterns

Isolated pisiform dislocation gets most of the attention in case reports, but combined injury patterns may be more clinically significant. The pisiform can dislocate alongside a triquetral fracture, creating a fracture-dislocation complex.18PubMed Central. An uncommon case of traumatic pisiform dislocation with triquetral fracture High-energy trauma can produce even broader patterns: one published case documented pisiform dislocation alongside both a distal radius fracture and a scaphoid fracture, with ulnar nerve injury on top of it all.19PubMed Central. Concomitant Distal Radius Fracture, Scaphoid Fracture, and Pisiform Dislocation with Ulnar Nerve Neuropraxia: A Case Report with Biomechanical Analysis

These combinations matter radiologically because the more dramatic injury can draw all the attention. A distal radius fracture is obvious and demands treatment. If the radiologist stops looking after finding it, the pisiform dislocation gets buried. This is a classic satisfaction-of-search error: the eye finds one abnormality and the brain relaxes its vigilance for a second one. When the mechanism of injury involves high energy, a systematic approach to reviewing every carpal bone on the CT is essential, regardless of what the dominant fracture is.

Chronic Pisotriquetral Problems in Athletes

Not every pisotriquetral injury presents as an acute dislocation. In batting sports like softball and baseball, repetitive impact through the ulnar side of the hand can produce a subtler pattern of pisotriquetral subluxation, impaction fracture, and eventual arthrosis. Two cases of adolescent female softball players with posttraumatic arthrosis and triquetral osteochondral nonunion associated with pisotriquetral subluxation were managed successfully with pisiform and triquetral fragment excision.20Journal of Hand Surgery. Posttraumatic Arthrosis and Triquetral Nonunion Associated With Pisotriquetral Subluxation in Adolescent Female Softball Players On imaging, these chronic cases show joint space narrowing, subchondral sclerosis, and sometimes loose osteochondral fragments in or near the pisotriquetral articulation. The clinical picture is chronic ulnar-sided wrist pain that worsens with grip-loading activities, and the diagnosis often comes late because the pisotriquetral joint is not on the standard mental checklist for athletic wrist pain.

When the Pisiform Comes Out Entirely

If closed reduction fails or the dislocation recurs, pisiformectomy (surgical excision of the bone) is the most commonly reported definitive treatment. The pisiform is a sesamoid, and removing it does not destabilize the carpal arch in the way that losing a scaphoid or lunate would. Long-term outcome data from a cohort of 57 patients who underwent pisiformectomy showed a median pain score of 0 after surgery and a median satisfaction score of 10 out of 10. The complication rate was about 13%, mostly involving transient ulnar nerve symptoms. Among patients who had ulnar nerve compression symptoms before surgery, the majority reported complete resolution afterward.21PubMed Central. Long-Term Results of Pisiformectomy in a Cohort of 57 Patients

From an imaging standpoint, post-pisiformectomy follow-up is straightforward: the bone is simply absent on subsequent radiographs or CT. What the radiologist needs to watch for in the postoperative period is not recurrence of dislocation but rather signs of complications, specifically any residual soft tissue mass, heterotopic ossification in the pisiform bed, or evidence of persistent ulnar nerve entrapment at Guyon’s canal. In patients who had associated triquetral fractures, follow-up imaging should also assess whether the triquetral fragment has healed or whether nonunion persists.

Avoiding the Missed Diagnosis

The biggest radiological pitfall with pisiform dislocation is not a failure of technology but a failure of suspicion. CT catches the injury reliably when it is ordered. The problem is that it often is not ordered, because the initial radiographs look reassuring and the clinical picture resembles a simple wrist sprain. One case report specifically highlighted how emergency physicians diagnosed a wrist sprain and discharged the patient, only for the pisiform dislocation to be identified on CT obtained later when symptoms did not resolve.22Journal of Surgical Case Reports. Isolated acute pisiform dislocation presenting atypically as wrist sprain in the emergency department

For radiologists reviewing acute wrist trauma, a few habits reduce the risk of missing this injury. Actively look at the pisiform on every wrist radiograph rather than skipping over the ulnar side once the radial structures appear normal. Request or recommend oblique views when ulnar-sided tenderness is prominent and the standard views are equivocal. When reading CT scans obtained for other wrist injuries, scroll through the pisotriquetral joint deliberately rather than focusing exclusively on the dominant fracture. And when reporting a confirmed dislocation, specify the direction of displacement and comment on the relationship to Guyon’s canal, because both details directly influence surgical planning.