Beyond the bones and joints: a review of ligamentous injuries of the foot and ankle on ^99mTc-MDP-SPECT/CT

Abstract

^99mTc-MDP-single photon emission computed tomography (SPECT)/CT has established itself as a useful modality in the assessment of the painful foot and ankle, owing to its ability to depict anatomical and functional information simultaneously. Despite its growing popularity, much of the literature describing the utility of ^99mTc-MDP-SPECT/CT of the foot and ankle is limited to osseous and articular pathology, with descriptions of ligamentous pathology limited to just a handful of cases. Though assessment of soft tissues using a combination of bone tracer and CT is limited, with a sound understanding of the regional anatomy, it is certainly within the scope of ^99mTc-MDP-SPECT/CT to accurately identify ligamentous injuries based on focal tracer uptake patterns and suspected ligamentous injuries should be reported as such. This article provides a case-based pictorial review of the ^99mTc-MDP-SPECT/CT appearances of the various ligamentous injuries of the foot and ankle with case examples and MRI correlation where available, most of which have yet to be described. The typical patterns of bone tracer uptake and associated CT abnormalities that may be observed are discussed.

Introduction

Ligamentous ankle sprains represent the most common site of injury amongst those engaged in sporting or recreational physical activities. Incidence is estimated as high as 27,000 injuries per day in the USA, and as many as 7–10% of emergency department attendances.^1–3 Ligamentous foot injuries are far less common, with Lisfranc injuries (including midfoot sprains and tarsometatarsal joint fracture-dislocations) accounting for only 0.2% of all fractures.⁴

Imaging plays a limited role in management of most lower extremity sprain injuries. In the context of ankle sprains, this is typically limited to plain radiographs in accordance with Ottawa Ankle Rules when examination findings are concerning for ankle fracture.⁵ When there is clinical concern of high-grade ligamentous injury, osteochondral injury or occult fracture, cross-sectional imaging is indicated, with MRI considered the gold standard given its high sensitivity and specificity.² However, use of MRI in the acute setting is limited given the high incidence of ankle sprains coupled with limited MRI resources.

While MRI is the diagnostic test of choice for foot and ankle ligamentous injuries, when clinical findings are less definitive and cause of pain remains uncertain, ^99mTc-MDP-single photon emission computed tomography (SPECT)/CT is not uncommonly performed to exclude occult fractures or other potential pain generators. ^99mTc-MDP-SPECT/CT has been gaining favour as a useful adjunct in imaging the painful foot and ankle in recent years owing to its ability to demonstrate anatomical, functional and mechanical information simultaneously.^6–9 However, most the literature describing the utility of ^99mTc-MDP-SPECT/CT is, unsurprisingly, in the context of osseous and articular pathology. Bone scintigraphy and its derivative techniques of SPECT and ^99mTc-MDP-SPECT/CT have traditionally been considered of limited value in assessing soft tissue pathologies. To date, only a few individual case examples of ankle ligamentous pathology on ^99mTc-MDP-SPECT/CT have been described, limited to the anterior talofibular ligament and syndesmosis.^7,8 Furthermore, at time of writing, we found no descriptions of ^99mTc-MDP-SPECT/CT concerning ligamentous foot injuries. Nevertheless, ^99mTc-MDP-SPECT/CT may provide the first, and sometimes only opportunity for a diagnosis ligamentous foot or ankle pathology to be made.

This article provides a comprehensive pictorial review of the ^99mTc-MDP-SPECT/CT appearances of the various ligamentous injuries of the foot and ankle as demonstrated by a case series of patients from our institution (Medical Imaging Consultants), many of which have yet to be described in the literature. Where available, correlation is made with corresponding MRI findings.

Syndesmotic ankle injury

Anterior-inferior tibiofibular ligament (AITFL) injury

Case 1. A 19 year-old male with a history of left ankle twisting injury while playing ice hockey and no radiographic evidence of fracture. ^99mTc-MDP-SPECT/CT of both ankles was performed 13 days post-injury with scintigraphic and associated subtle CT features of AITFL injury described in Figure 1. MRI obtained 18 days later confirmed complete AITFL tear (Figure 2). It should be noted that with this case and many of the examples that follow, the clinical concern was to exclude radiographically occult fracture. Hence, ^99mTc-MDP-SPECT/CT was initially requested given its high accuracy in detecting occult fractures, and faster accessibility than MRI in our regional healthcare service. MRI was subsequently obtained to confirm the ligamentous injuries suspected on ^99mTc-MDP-SPECT/CT.

Figure 1. .

(a) Axial (b) coronal and (c) sagittal oblique fused ^99mTc-MDP SPECT/CT reconstructions of the left ankle in a 19 year-old male (case 1) demonstrate linear increased tracer uptake extending obliquely from the anterolateral margin of the tibia to the anterior fibula in the expected location of the AITFL. Interestingly, tracer uptake is not limited to the bone, but is also seen to span the intervening soft tissues in a linear fashion. There is no associated fracture or cortical irregularity, although the axial CT component of both ankles (d) shows subtle relative widening of the anterior tibiofibular interval on the left (red arrow) compared to the non-injured side, supporting a diagnosis of AITFL injury. AIFTL, anterior inferior talofibular ligament; SPECT, single photon emission computed tomography.

Figure 2. .

Corresponding MRI from case 1 acquired 18 days following the ^99mTc-MDP SPECT/CT study. (a) Coronal PD fat sat and (b) axial PD MRI images confirm complete tear of the AITFL from its fibular attachment. AIFTL, anterior inferior talofibular ligament; SPECT, single photon emission computed tomography

Posterior-inferior tibiofibular ligament (PITFL) injury

Case 2. A 38 year-old male with a history of fall and landing awkwardly on the left foot with unremarkable initial radiographs. ^99mTc-MDP-SPECT/CT was performed 6 weeks post-injury to exclude radiographically occult fracture, demonstrating features of PITFL and subtle AITFL injury (Figure 3a-c). MRI 3 months later confirmed partial stripping injury of the PITFL tibial attachment (Figure 3d,e). MRI also confirmed sprain of the AITFL.

Figure 3. .

(a) Axial (b) coronal oblique and (c) sagittal oblique fused ^99mTc-MDP SPECT/CT reconstructions of the left ankle in a 38-year-old male (Case 2) demonstrate focal intense tracer uptake localising to the posterolateral distal tibia at the expected attachment site of the PITFL without associated fracture or osseous abnormality on the CT component. Given the associated focal hyperaemia in this region on corresponding initial blood pool images (not shown), this was initially reported as a probable stress fracture occult on CT. Low grade focal increased tracer uptake at the expected region of the AITFL (red arrow in A) was considered likely due to AITFL injury. Corresponding (d) axial STIR and (e) axial PD MRI images acquired 3 months later demonstrate focal bone marrow oedema at the tibial attachment of the PITFL, consistent with partial ligamentous stripping. Localized reactive bone marrow oedema accounts for the focal activity seen on ^99mTc-MDP-SPECT/CT. The AITFL is also attenuated and irregular with focal bone marrow oedema at the tibial attachment, confirming injuries to both the AITFL and PITFL. There was no MRI evidence of fracture. AIFTL, anterior inferior talofibular ligament; PITFL, posteriorinferior talofibular ligament; SPECT, single photon emission computed tomography; STIR, short tau inversion-recovery.

Lateral collateral ligamentous complex injury

Anterior talofibular ligament (ATFL) injury

Case 3. A 28 year-old female with a history of left ankle inversion injury. ^99mTc-MDP-SPECT/CT performed 1 month later demonstrated localized tracer uptake in keeping with ATFL injury without CT osseous abnormality (Figure 4).

Figure 4. .

(a) Coronal fused (b) corresponding coronal CT (c) axial oblique fused and (d) corresponding axial oblique CT ^99mTc-MDP SPECT/CT reconstructions of the left ankle in a 28-year-old female (Case 3). There is focal increased tracer uptake localising to the lateral talar process without osseous abnormality at the expected attachment of the ATFL, consistent with ATFL injury. AFTL, anterior talofibular ligament; SPECT, single photon emission computed tomography.

Posterior talofibular ligament (PTFL) injury

Case 4. A 37-year-old male with recent left lateral ankle pain but no history of trauma. ^99mTc-MDP-SPECT/CT performed 2 months post-pain onset demonstrated tracer uptake suggestive of PTFL injury without CT osseous abnormality (Figure 5a-c). MRI performed 2 weeks later confirmed PTFL sprain and associated ATFL and CFL injuries occult on ^99mTc-MDP-SPECT/CT (Figure 5d-f).

Figure 5. .

(a) Coronal (b) axial and (c) sagittal fused ^99mTc-MDP SPECT/CT reconstructions of the left ankle in a 37-year-old male (Case 4) demonstrate focal increased tracer uptake localizing to posterolateral margin of the talar body and the adjacent malleolar fossa of the lateral malleolus in the expected location of the PTFL. Corresponding (d) coronal PD fat sat and (e) axial STIR MRI images acquired 2 weeks later show interleaved intrasubstance oedema within the thickened PTFL with associated extensive bone marrow oedema within the distal lateral malleolus, confirming PTFL sprain. As is typically seen in cases of PTFL sprain, MRI additionally showed associated sprains of the ATFL and CFL that were not identified on ^99mTc-MDP SPECT/CT, the former can be seen in (e) where there is thickening and periligamentous oedema of the ATFL. A small posterior ankle joint effusion and mild synovial hypertrophy is seen in association. AFTL, anterior talofibular ligament; CFL, calcaneofibularligament; PTFL, posterior talofibular ligament; SPECT, single photonemission computed tomography.

Calcaneofibular ligament (CFL) injury

Case 5. A 34-year-old female with recurrent right lateral ankle pain following twisting injury. ^99mTc-MDP-SPECT/CT performed 3 months post-injury demonstrated scintigraphic features of CFL, as well as ATFL and AITFL injury without CT osseous abnormality (Figure 6a-c), and was reported as extensive lateral ankle and probable AITFL ligamentous injury. MRI performed 8 days later demonstrated complimentary findings of CFL and ATFL sprain (Figure 6d,e), although the AITFL was unremarkable.

Figure 6. .

(a) Coronal oblique (b) sagittal oblique and (c axial fused ^99mTc-MDP SPECT/CT reconstructions of the right ankle in a 34-year-old female (Case 5) demonstrate a striking pattern of multifocal lateral ankle increased tracer uptake conforming to ligamentous attachment sites without focal osseous abnormality on the CT component. A broad linear band of focal tracer uptake extends obliquely inferomedially from the distal tip of the lateral malleolus to the lateral calcaneal margin (a, b) conforming to the expected course of the CFL. Tracer uptake at the anterolateral tibia and adjacent fibula (a), and extending from the anterior margin of the lateral malleolus to the lateral talus in (c) conform to the expected locations of the AITFL and ATFL respectively. Corresponding (d) coronal PD fat sat and (e) axial STIR MRI images acquired 8 days later confirm sprains of the CFL (d), which appears attenuated with periligamentous oedema, and thickening of the ATFL with periligamentous oedema (e), although there was no MRI evidence of injury to the AITFL with no corresponding MRI finding to account for increased tracer uptake seen on ^99mTc-MDP SPECT/CT. AFTL, anterior talofibular ligament; AIFTL, anterior inferior talofibular ligament; CFL, calcaneofibularligament; PTFL, posterior talofibular ligament; PITFL, posterior inferiortalofibular ligament; SPECT, single photon emission computed tomography; STIR,short tau inversion-recovery.

Deltoid ligament injury

Case 6. A 36-year-old female with persistent left ankle pain following ankle eversion. ^99mTc-MDP-SPECT/CT performed 3 months post-injury demonstrated tracer uptake conforming to the deep deltoid ligament with associated cortical avulsion on CT. (Figure 7). The study was reported as an avulsive injury to the deep deltoid ligaments.

Figure 7. .

(a) Planar frontal view of both ankles and subsequent (b) axial fused (c) coronal fused (d) sagittal oblique fused and (e) sagittal oblique CT ^99mTc-MDP SPECT/CT reconstructions of the left ankle in a 36-year-old female (Case 6). Focal intense increased tracer uptake at the medial ankle on planar studies correlates with intense tracer uptake at the lateral margin of the inferior medial malleolar tip on SPECT/CT, extending posteromedially through the soft tissues in a linear fashion to terminate at the medial talar margin at the expected location of the deep deltoid ligament. At the epicentre of increased tracer uptake there is a small avulsed cortical flake arising from the adjacent medial malleolar ligamentous attachment, consistent with avulsive injury. SPECT, single photon emission computed tomography.

Ligamentous injuries of the foot

Lisfranc ligament complex injury

Case 7. A 53-year-old male with persistent pain in the right first metatarsal region for 3 months, no recollection of trauma and unremarkable foot radiographs. ^99mTc-MDP-SPECT/CT was performed to exclude radiographically occult fracture, demonstrating uptake conforming to the location of the Lisfranc ligament with subtle associated subcortical fracture line on CT (Figure 8). The study was reported as avulsion injury of the Lisfranc ligament.

Figure 8. .

Planar (a) medial view of the right foot (b) plantar view of both feet in a 53-year-old male (Case 7) show focal increased tracer uptake approximately between the right first and second tarsometatarsal joints, corresponding to the site of tenderness. Subsequent (c) axial fused (d) axial CT (e) coronal fused and (f) coronal CT ^99mTc-MDP SPECT/CT reconstructions of the right foot demonstrate focal increased tracer uptake localizing to the medial margin of the base of second metatarsal and adjacent lateral margin of the distal medial medial cuneiform. Non-fused CT reconstructions (d, f) show a subtle non-displaced fracture line of the metatarsal base corresponding to the epicentre of increased tracer uptake, consistent with non-displaced avulsion injury of the Lisfranc ligament complex. The planar studies show additional increased tracer uptake in the calcaneal region, which corresponded to the plantar fascia calcaneal attachment on ^99mTc-MDP SPECT/CT (not shown), consistent with incidental plantar fasciitis. SPECT, single photon emission computed tomography.

Bifurcate ligament injury

Case 8. A 33-year-old male with persistent pain localising to the right cuboid for 3 months, no trauma history and unremarkable foot radiographs. ^99mTc-MDP-SPECT/CT was performed to exclude radiographically occult stress fracture, demonstrating localised tracer uptake conforming to the expected location of the bifurcate ligament and no fracture component on CT (Figure 9a-d). This corresponded to the site of pain and was reported as probable bifurcate ligament injury. MRI was performed 3 days later for clarification, demonstrating focal bone marrow oedema conforming to the bifurcate ligament attachments, consistent with tug-type ligamentous injury (Figure 9e,f).

Figure 9. .

(a) Sagittal oblique fused (b) sagittal oblique CT (c) axial fused and (d) axial CT ^99mTc-MDP SPECT/CT reconstructions of the right foot in a 33 year-old male (case 8). The epicentre of increased tracer uptake localizes to the junction of the anterior calcaneal process and the adjacent cortices of the medial cuboid and lateral navicular, in the expected location of the bifurcate ligament, without osseous abnormality on the CT component. This corresponded to the site of pain. Corresponding (d) sagittal STIR and (e) axial STIR MRI images acquired 3 days later show complimentary findings of focal mild bone marrow oedema of the anterior calcaneal process and adjacent cuboid and navicular, although the bifurcate ligament itself is intact, consistent with insertional ligamentous tug-type injury. SPECT, single photon emission computed tomography; STIR, short tau inversion-recovery.

Conclusion

The paucity of existing literature describing ^99mTc-MDP-SPECT/CT findings in foot and ankle ligamentous pathology is perhaps surprising, particularly given the frequency with which ankle sprain injuries occur. At time of writing, we found no other articles describing ^99mTc-MDP-SPECT/CT findings in the context of CFL, PTFL, deltoid, Lisfranc or bifurcate ligament injuries described above.

This is perhaps in part, due to the complexity of the ligamentous anatomy coupled with their limited visualisation on CT. The ligaments are often difficult, if not impossible, to discern as discrete structures on CT, such that their location and associated injuries must often be inferred from the surrounding osseous anatomy.¹⁰ As depicted in the above examples, while some ligamentous injuries may be accompanied by associated osseous avulsion (Case 6) or widening of the bony interval due to ligamentous incompetence (Case 1), many do not result in demonstrable CT osseous abnormality.¹⁰ Care must therefore be taken not to dismiss any focal tracer uptake without corresponding osseous abnormality as simply a site of stress reaction but rather, to determine whether focal uptake may correspond to a site of soft tissue osseous reaction such as a ligamentous attachment. The case examples with complementary MRI findings also exemplify the approximate congruence between foci of scintigraphic bone activity and bone marrow oedema seen on MRI.^11,12 As such, it is our opinion that with a sound understanding of the regional ligamentous anatomy, the increased osteoblastic activity elicited by ligamentous injuries can often be accurately depicted using ^99mTc-MDP-SPECT/CT, but is nevertheless easily overlooked or misinterpreted.

The lack of literature describing ^99mTc-MDP-SPECT/CT findings in foot and ankle ligamentous pathology may also be in part due to reporting practices.

^99mTc-MDP-SPECT/CT as a modality has evolved from planar-bone scintigraphy and SPECT, both of which have limited application in soft tissue assessment.¹³ Detailed ligamentous pathology is beyond the scope of these purely scintigraphic modalities, which lack sufficient spatial resolution, anatomic correlation or specificity to confidently localise and characterize tracer uptake to such a level of detail.¹³ While the additional CT component brings a wealth of additional soft tissue and anatomical information that makes diagnosis of ligamentous and other soft tissue pathologies possible, reporting practices have not necessarily evolved to reflect this. Current European Association of Nuclear Medicine ^99mTc-MDP-SPECT/CT practice guidelines also make limited references to soft tissue applications of ^99mTc-MDP-SPECT/CT in trauma/orthopaedic or any other clinical context, limited only to plantar fasciitis, Achilles tendinitis, bursitis, and assessment of soft tissue tumours (Table 1).¹⁴

Table 1. .

Orthopaedic, sport and traumatology indications for ^99mTc-MDP-SPECT/CT adapted from EANM 2016 guidelines. The list of potential indications for ^99mTc-MDP-SPECT/CT is vast, includes numerous oncology, rheumatology, bone/joint infection, metabolic bone disease and paediatric applications further detailed in the EANM 2016 guidelines.¹⁴

Periostitis	Includes shin splints, thigh splints
Enthesopathies	Plantar fasciitis, Achilles tendinitis and bursitis
Spondylolisthesis	Acute or subacute
Fractures	Radiologically occult stress-related
	Insufficiency fractures (osteoporotic or occult fractures)
Internal fixation/prosthesis complications	Septic loosening, mechanical complication, synovitis
Pseudoarthrosis	Delayed union, non-union
Periarticular heterotopic ossification
Viability of bone graft

The list of potential indications for ^99mTc-MDP-SPECT/CTis vast, includes numerous oncology, rheumatology, bone/joint infection, metabolic bone disease and paediatric applications further detailed in the EANM 2016 guidelines.¹⁴

Ultimately, MRI remains the imaging modality of choice to demonstrate ligamentous pathology of the foot and ankle.¹⁵ Given that the relative sensitivity of ^99mTc-MDP-SPECT/CT for ligamentous injuries is unknown (particularly low-grade injuries less likely to elicit an osseous reaction), early ^99mTc-MDP-SPECT/CT cannot be considered a substitute for delayed MRI. However, the above examples demonstrate that with good understanding of the ligamentous anatomy, it is certainly within the scope of ^99mTc-MDP-SPECT/CT to accurately identify such pathology, with distribution of tracer uptake typically approximating to bone marrow oedema on MRI. By virtue of its CT component, bony detail and associated osseous abnormalities will also often be more easily seen on ^99mTc-MDP-SPECT/CT. Given that most patients with prior foot and ankle ligamentous injuries have undergone little more than plain radiographs, ^99mTc-MDP-SPECT/CT may often provide the first and sometimes only opportunity to identify incidental ligamentous pathology, and as such, uptake patterns suggesting ligamentous injury should be recognized and reported.