Loose Body in the Knee

Definition. An intra-articular loose body is a fragment of bone, cartilage, or osteocartilage lying free (or pedunculated) within the synovial cavity, capable of moving between recesses and of causing mechanical symptoms. Three things have to be true of a fragment before it earns the name.

It is inside the capsule. On the lateral view the suprapatellar pouch, the intercondylar notch and the posterior recesses are the resting places. A posterolateral ossicle superimposed behind the lateral femoral condyle and lying within the gastrocnemius tendon shadow is almost always the fabella: a sesamoid in the lateral head of gastrocnemius, outside the capsule, present in about one in five knees and often bilateral, whose position relative to the condyle is constant on every film. The cyamella, in popliteus, is the rarer cousin.

It moves. Compare with any prior radiograph, or repeat the lateral in a different degree of flexion. A change in position between studies is the single most specific plain-film sign. If the position is fixed and typical you have finished: this is a sesamoid or an osteophyte and needs nothing further.

It is not attached. Osteophytes are contiguous with the joint margin, so look for a stalk of cortex. A fragment fully surrounded by lucency on two orthogonal views is free.

Saying it out loud. A report that names the fragment, the donor bed and the next test:

"This is an AP and lateral radiograph of a skeletally immature left knee. There is a well-corticated ovoid ossific fragment measuring approximately 12 millimetres lying in the intercondylar notch. There is a corresponding lucent defect with a sclerotic base on the lateral aspect of the medial femoral condyle on the tunnel view. Joint space is preserved and there is a moderate effusion. The appearances are those of a detached osteochondritis dissecans fragment — a loose body — and I would confirm with MRI to assess the donor bed, the stability of any remaining lesion, and to look for further chondral bodies."

Mimics. The false positives, and what gives each away:

Plain radiographs, always first. Request four views:

• AP and lateral
• Skyline, for post-dislocation fragments
• Tunnel (Rosenberg or intercondylar), essential for OCD of the lateral aspect of the medial femoral condyle

The films confirm ossification, position, cortication, joint space and the presence of a donor bed. They change management by identifying secondary osteoarthritis, in which case symptomatic bodies are treated arthroscopically or the joint is managed as OA. What they cannot do is exclude a loose body: around a third to a half are purely cartilaginous and radiolucent.

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When to stop. No further imaging is needed when the fragment is a fabella, a cyamella, a bipartite patella or an attached marginal osteophyte in an asymptomatic knee, or when established osteoarthritis with multiple bodies is evident and management is non-operative regardless. Reassure and stop.

MRI, the workhorse. Indicated for any suspected chondral-only body, a locked knee with normal radiographs, suspected OCD staging, or a soft tissue mass. It shows the donor bed, the signs of an unstable OCD fragment listed in the differential above, meniscal tears, ligament injury and the haemosiderin blooming of PVNS. It changes management by distinguishing a stable OCD lesion (non-operative, restricted loading) from an unstable or detached one (fixation, or removal plus marrow stimulation or graft).

CT and CT arthrogram. Best for characterising ossified bodies, counting them before arthroscopy, and mapping the donor bed geometry when MRI is contraindicated or metal artefact degrades it. A CT arthrogram outlines radiolucent chondral bodies. In synovial chondromatosis it determines how many bodies there are and how far into the posterior recesses they extend, which decides whether an open or posterior portal approach is needed.

Joint aspiration. Mandatory if infection is possible. Fat globules in a haemarthrosis indicate an intra-articular fracture, and crystals confirm gout or pseudogout as an alternative cause of a painful effusion with calcification.

Staged biopsy, never incidental. A soft tissue mass goes to a sarcoma unit, and the biopsy is taken through a planned incision, not the arthroscopy portal. An unplanned arthroscopy of a synovial sarcoma contaminates all portals and compartments and may convert a limb-salvage case into an amputation.

Arthroscopy. Both diagnostic and therapeutic for the locked knee, mechanical symptoms, or bodies confirmed on imaging. In primary synovial chondromatosis, phase the disease at arthroscopy rather than applying one rule (Milgram): in phase 3, free bodies with a quiet synovium, removal alone may suffice, while in phases 1 and 2 a synovectomy is reasonable but does not guarantee against recurrence, because the cartilaginous lesions that regrow may already lie outside the synovium. Document all recesses, including posteromedial and posterolateral.

Related pages. The causes, in order of the differential: Osteochondritis Dissecans of the Knee is the commonest true loose body in the young knee, Synovial Chondromatosis is the multiple-uniform-bodies diagnosis, and Spontaneous Osteonecrosis of the Knee is its weight-bearing mimic in the older patient.

The list runs from a sesamoid to a sarcoma. The table sorts it with the diagnoses a miss would cost most at the top; the next section is how to walk it against the film in front of you.

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Once the fragment is real, six questions take it to a diagnosis. Set the differential by age before interpreting the fragment itself.

How old is the patient? Under 20 with a single body: osteochondritis dissecans or an acute osteochondral fracture. Between those ages, multiple bodies mean primary synovial chondromatosis (30-50) or, if there is no calcification, PVNS (20-50). Over 55 with multiple irregular bodies and joint space loss: osteoarthritis or SONK. Age alone removes half the list.

Solitary or multiple, and are they the same size? Count the bodies and compare their sizes and internal matrix with one another. A solitary body points to OCD, osteochondral fracture or ACL avulsion. Multiple bodies of uniform size with a chondroid ring-and-arc pattern is primary synovial chondromatosis; multiple bodies of varying size in a degenerate joint is secondary chondromatosis. This single question separates the two chondromatoses.

Where is the donor defect within the bone? Find the bed the fragment came from, and name its exact location rather than the bone. The lateral aspect of the medial femoral condyle, non-weight-bearing beside the notch, is classic OCD; the weight-bearing zone of the medial femoral condyle in an elderly patient is SONK; the patellar median ridge or lateral trochlea is a post-dislocation osteochondral fracture; the anterior intercondylar eminence is an ACL avulsion. No defect at all means the origin is synovial. Reporting the fragment without describing where it came from loses the diagnosis, and the tunnel view for OCD and the skyline for a post-dislocation fracture are both needed before you say the origin is unknown.

Is the fragment corticated or sharp? Inspect the margins and ask whether the fragment would jigsaw back into a matching bed. Well corticated with smooth rounded margins means chronic: it has been free long enough to be nourished by synovial fluid, remodel, and possibly grow. A sharp non-corticated margin with a matching bed means an acute fracture, and changes the plan to urgent fixation if the fragment is large and the bed is fresh. In children after patellar dislocation nearly half of these fractures are invisible on the plain film (44 per cent radiograph-negative in Seeley's series), so a normal radiograph does not settle it.

Is there a soft tissue mass or a systemic feature? Look beyond the fragment at the soft tissues, and check temperature, CRP and the history of effusions. A juxta-articular soft-tissue mass with faint punctate calcification and bone erosion in a 15-40 year old is synovial sarcoma until proven otherwise and demands a sarcoma work-up before any arthroscopy; synovial chondromatosis gives discrete ring-and-arc chondroid bodies inside the capsule, not an amorphous mass eroding bone. Fever, a raised CRP and an effusion demand aspiration. Recurrent atraumatic haemarthrosis with erosions and a preserved joint space suggests PVNS, and painless gross debris suggests a neuropathic joint.

Does the knee have a true mechanical block? Test passive extension yourself rather than accepting the word "locking" from the history; true locking is a sudden mechanical block that the patient has to manoeuvre out of. A knee that cannot be passively extended is a surgical indication irrespective of the radiograph. If the film is normal the culprit is most often a bucket-handle meniscal tear or a purely chondral body: go to MRI, then arthroscopy.