Clinical History
This 42-year-old female patient was admitted to our institution reporting pain and swelling on the ball of her left foot, especially localised at the metatarsophalangeal joint of the second toe. The patient was a flight-assistant and she reported repetitive use of high-heels shoes. MR and US evaluation were performed.
Imaging Findings
MR examination of the left foot, performed with a low-field (0.18T) extremity-dedicated MRI system, revealed oedematous thickness of soft tissues surrounding the second toe metatarsophalangeal (MTP) joint, with signal alteration on the plantar side, in keeping with plantar plate tear (Fig. 1). An ultrasound examination was performed to better investigate the MTP joint: US evaluation showed a hypo-echoic defect on the lateral side of the plantar plate, confirming the presence of a partial tear in the distal part of the plantar plate (Fig. 2).
Discussion
The plantar plate is a fibrocartilaginous structure localised near the metatarsophalangeal (MTP) joint; it has a proximal attachment to the periosteum of the metatarsal diaphysis and a distal insertion directly to the plantar side of the proximal phalanx. The plantar plate has a central position and presents multiple important attachments to the collateral ligaments, to the plantar fascia, to the inter-metatarsal ligaments and to the fibrous sheath of the flexor tendons: as a consequence, it is easy to understand the important bio-mechanical role of the plantar plate as a central stabiliser of the metatarsophalangeal joint. Static stabilisation of the MTP joint is primarily function of the plantar plate, while dynamic stabilisation is provided by the extrinsic and intrinsic muscles of the forefoot, and their ability to stabilise the joint depends on the integrity of the plantar plate. After rupture of the plantar plate, the proximal phalanx tends to subluxate dorsally, and the extensor tendons are unable to extend the interphalangeal joints. MTP joint instability presents clinically with pain in the forefoot; symptoms may include swelling, malalignment, neuritic symptoms and dysfunction in the normal articular bio-mechanics of the foot. MTP joint instability can be associated with extra- and intra-articular conditions, and the differential diagnosis comprehends pathologies such as Morton's neuroma, synovial cysts or Freiberg’s infraction [1].
At MR imaging, features of plantar plate rupture correspond to substitution of the normal hypo-intense area with a focal hyper-intense finding. Sagittal images can provide a better evaluation of the thickness of the tear, while coronal images allow establishing the position of the lesion [2]. It is important to underline that only high-field MR can provide a diagnosis and it should be considered the gold standard in evaluation of plantar plate ruptures; however, in selected cases, US examination can add insightful information to achieve a complete understanding of the MTP joint condition. At US rupture of plantar plate presents as a loss of the normal homogeneous structure and tears appear as little hypoechoic defects in the context of distal insertional fibers: tears could occur as partial-thickness lacerations or as full-thickness lacerations. Most of the tears occur at the distal attachment on the proximal phalanx [3].
In conclusion, a correct clinical approach and a diagnostic iter in which dynamic US can play a considerable role are fundamental to achieve a correct diagnosis of metatarsophalangeal joint instability, a condition often unrecognised and potential source of frustration for the orthopaedic surgeon.
Differential Diagnosis List
Plantar plate partial-thickness tear
Synovial cysts
Freiberg\'s infraction
Interdigital neuroma
Final Diagnosis
Plantar plate partial-thickness tear
