How Weightbearing CT Can Be A Valuable Diagnostic Tool

Author(s): 
Kent Feldman, DPM

This author discusses the case of a 63-year-old with forefoot pain, noting how a weightbearing computed tomography (CT) scan aided in her diagnosis by providing accurate measurements of the metatarsal heads.

When evaluating the patient with metatarsalgia, there are many anatomic and biomechanical factors to consider: first ray hypermobility, metatarsal length, metatarsal elevation and metatarsophalangeal joint (MPJ) stability (plantar plate). One may use traditional weightbearing radiographs to evaluate metatarsal length but they cannot assess metatarsal sagittal plane position. Sesamoid axial views can evaluate the metatarsal head sagittal plane position but such radiographs are not reliable as the patient or the X-ray technician positions the foot in a way that clearly does not represent a resting stance position.

   Weightbearing computed tomography (CT) can accurately measure the relative position of the metatarsal heads to one another and to the weight bearing surface.

A Closer Look At The Patient Presentation

A 63-year-old female presents with persistent left forefoot pain. She points to the plantar margin of the third metatarsal head as the area of discomfort and pressure. She has had four previous foot surgeries by another provider. These surgeries included a first metatarsal base osteotomy, a revision of the base osteotomy with bone grafting and a first metatarsal head osteotomy. She has also had a second metatarsal head osteotomy.

   Clinically, the neurovascular and dermatological exams are normal with the exception of a small callus beneath the left third metatarsal head. The plantar fat pad is thin beneath the third metatarsal head. The second metatarsal is shortened and elevated, and does not bear weight. In a relative sense, the third metatarsal head is plantarly prominent. The second MPJ is extended and she has poor flexor strength within the second toe. The third MPJ is slightly extended.

What The Weightbearing Foot CT Scan Reveals

The transverse slices of the weightbearing CT scan allow for the precise evaluation of metatarsal length. Depending on the technique for determining metatarsal parabola and the ideal postoperative position, the surgeon can accurately measure metatarsal length. The sagittal weight bearing slices can identify the position of the metatarsal heads in relation to the ground and one another.

   In this case, the second metatarsal head is elevated off the weightbearing surface 9.10 mm (Figure 2) while the third metatarsal head is elevated 5.60 mm (Figure 3) and the fourth metatarsal is elevated 5.42 mm (Figure 4). The second metatarsal is 2.7 mm shorter in length than a line between the first and thirrd metatarsals and the third metatarsal is 4.95 mm longer than a line between the second and fourth metatarsals (Figure 5).

A Guide To Surgical Planning

The surgical goal is to even out the metatarsal weightbearing pattern to reduce the sub-third metatarsal pressure, pain and callus formation. One option would be to add length and plantarly transpose the second metatarsal head. This would require a plantarflexing osteotomy with slide lengthening of the metatarsal base or a metatarsal head osteotomy with a bone graft. As the patient had had difficulties healing a first metatarsal base osteotomy (requiring bone grafting), it made better sense to shorten and elevate the third metatarsal.

   Taking the weightbearing CT measurements into consideration, the goal would be to shorten the third metatarsal by 4.95 mm and elevate the third metatarsal approximately 2 mm. This would make the third metatarsal the same length as the second and fourth metatarsals and place the third metatarsal in a position of elevation between the second and fourth metatarsals. Other surgeons might consider alternative approaches. The weightbearing CT scans allow the surgeon to create a patient and pathology specific surgical plan.

   I treated the second digit with an extensor tendon lengthening and a flexor tendon transfer in an attempt to gain digital purchase. The third metatarsal had an oblique head osteotomy parallel to the weightbearing surface. I shortened the metatarsal head with a slide technique. I elevated the metatarsal head by removing a thin wafer of bone from the osteotomy site. The postoperative course was uneventful. The patient has normal digital alignment and no longer has sub-third metatarsal pain.

In Conclusion

The weightbearing CT study provides the surgeon with the critical anatomic information to allow for precise surgical planning. One can use this information to lengthen and plantarflex the second metatarsal head, or to shorten and elevate the third metatarsal head. This information may also help conservatively manage patients with altered metatarsal weightbearing patterns.

   Dr. Feldman is the podiatry provider for the San Diego Chargers and the Departments of Athletic Medicine at San Diego State University, Point Loma Nazarene University and Christian Heritage College. He is board certified in foot surgery by the American Board of Podiatric Surgery.

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