Blood investigations
Blood tests aim to diagnose inflammatory arthropathy, peripheral neuropathy or diabetes.
If the clinical picture suggests inflammatory arthritis appropriate tests would include:
- full blood count
- ESR
- CRP
- rheumatoid serology
However, although a few patients with lesser ray pain have positive inflammatory markers, very few have strongly positive rheumatoid serology, and those who do usually have other evidence of rheumatoid disease such as a more generalised symmetrical polyarthritis, marked and prolonged early morning stiffness, typical toe deformities or nodules. There are no studies looking at the diagnostic value of such tests, but unless there are other reasons to suspect inflammatory arthritis it is probably not worth doing them.
Psoriatic arthropathy may present in the foot, sometimes with few or occasionally no psoriatic plaques.
If there are abnormal neurological signs suggestive of a peripheral neuropathy such as a stocking-type sensory deficit, a review article in the British Medical Journal (Hughes 2002) recommended:
- first line tests:
- urine glucose and protein
- full blood count
- ESR
- vitamin B12 and folate levels
- fasting glucose
- renal and liver function tests
- thyroid function tests
- second line test (if diagnosis not made from initial tests)
- nerve conduction studies
- protein electrophoresis
- serum angiotensin converting enzyme
- ANF
- antiextractable nuclear antigen antibodies (anti-Ro, anti-La)
- antineutrophil cytoplasmic antigen antibodies
- chest radiography
A neurology consultation may be helpful, and it is best to agree a pre-referral investigation protocol to avoid wasting the neurologist's time.
Undiagnosed diabetes presenting as forefoot pain would usually be part of the differential diagnosis of peripheral neuropathy and would usually be apparent on urine testing or a fasting blood glucose. A formal glucose tolerance test may occasionally be required.
Radiology
Imaging aims to:
- identify underlying disease such as erosive arthropathy
- assess deformity
- measure metatarsal length and position where this is thought clinically to be a significant contributing factor to pain.
Remember that plain radiographs give a two-dimensional projection of the metatarsal position, and views in different planes may be necessary to appreciate an abnormal metatarsal alignment or position.
Standard plain radiographic views include:
- standing AP/lateral forefoot - some surgeons also include an oblique view but there is no data on how much information this adds
- metatarsal skyline view if plantar condylar eminence suspected
- "deformity series" (standing hindfoot alignment view, standing lateral foot, standing dorsoplantar foot) in the presence of overall foot deformity such as pes cavus
There are at least three methods of measuring comparative metatarsal lengths, all of which have been used in numerous clinical studies. There are no studies comparing them as predictors of outcome from surgery. Chauhan (2008) compared the relative protrusion of the second metatarsal when measured with the Coughlin, Maestro and Hardy methods. The Coughlin method gave the largest protrusion with the highest reproducibility; te Maestro method was similar. However, the Hardy/Clapham method gave a much lower value and was least reproducible. Moreover, Bevernage (2008) found that achieving planned shortening of Weil osteotomies did not give better results than metatarsals in which planned length was not achieved.
Ultrasound and MRI are good at early identification of erosions in the MTP joints and may allow inflammatory arthritis to be identified and treated early enough to prevent major joint destruction.
Both ultrasound and MRI are sensitive and specific in identifying interdigital neuromas and interdigital bursitis, although the diagnosis is usually apparent clinically without imaging.
Foot pressure studies
The optical pedobarograph and numerous commercial electronic pressure measuring systems both in-shoe and outside the shoe can give a considerable amount of information on pressure distribution.
However, they can be difficult to calibrate and register with the foot itself. Data from one technique is not necessarily compatible with data from another. In addition, the range of normal is very wide. Few studies have shown clear clinical value in foot pressure studies, but they may be very useful in research.