An Association Between Functional Second Metatarsal Length and Midfoot Arthrosis

Davitt, James S. MD; Kadel, Nancy MD; Sangeorzan, Bruce J. MD; Hansen, Sigvard T. Jr. MD; Holt, Sarah K. MPH; Donaldson-Fletcher, Emily BA

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.C.01238
Scientific Articles
Abstract

Background: Primary tarsometatarsal arthrosis is relatively uncommon. The etiology of osteoarthritis in the foot is poorly understood, and it is possible that mechanical or anatomic factors play a role.

Methods: We compared the relative length of the metatarsals in patients with idiopathic arthrosis of the midfoot with that in a group of controls without arthrosis. We analyzed the radiographs of all patients who had had an arthrodesis of the first, second, and third tarsometatarsal joints to treat arthrosis during a three-year period at a tertiary teaching hospital. We excluded patients with a history of inflammatory arthritis, trauma, or Charcot arthropathy. Nine patients (fifteen feet), seven women and two men with an average age of 64.2 years, met the inclusion criteria. We compared them with a control group consisting of the uninjured feet of patients with an acute traumatic injury to the hindfoot and the feet of volunteers with no foot problems. We measured the first, second, and fourth metatarsal lengths and the intermetatarsal angles on weight-bearing anteroposterior radiographs. We also measured the length of the first metatarsal relative to the long axis of the second metatarsal to define the functional first metatarsal length. The ratios of metatarsal lengths and the ratios of functional lengths were used for analysis to minimize differences in foot size and differences caused by radiographic magnification. Statistical comparisons between groups were then carried out.

Results: In the study group, the length of the first metatarsal was, on the average, 77.0% of the length of the second metatarsal, whereas, in the control group, the first metatarsal length was an average of 82.0% of the second metatarsal length. The functional length of the second metatarsal was, on the average, 18.6% greater than that of the first metatarsal in the study group and only an average of 4.1% greater than that of the first metatarsal in the control group. Both differences were significant (p < 0.0004 and p < 0.0001, respectively).

Conclusions: Patients with midfoot arthrosis had a different ratio of the first to the second metatarsal length than did a similarly aged cohort without midfoot arthrosis. The patients had a relatively short first metatarsal or a relatively long second metatarsal, or both. Midfoot arthrosis may have a mechanical etiology. Recognition of risk factors is the first step in developing prevention strategies.

Level of Evidence: Prognostic Level III. See Instructions to Authors for a complete description of levels of evidence.

Author Information

1 Orthopedic and Fracture Clinic, 11782 S.W. Barnes Road, Suite 300, Portland, OR 97225. E-mail address: james.davitt@providence.org

2 Department of Orthopaedics and Sports Medicine, University of Washington, Harborview Medical Center, Box 359798, 325 Ninth Avenue, Seattle, WA 98104. E-mail address for N. Kadel: kadel@u.washington.edu. E-mail address for B.J. Sangeorzan: bsangeor@u.washington.edu. E-mail address for S.T. Hansen Jr.: jegrant@u.washington.edu. E-mail address for S.K. Holt: sholt@u.washington.edu

3 E-mail address: donaldea@whitman.edu

Article Outline

Arthrosis in the foot and ankle is a common cause of disability and pain. Usually it is the result of prior trauma or is secondary to inflammatory arthritis, with inflammatory arthritis predominating in the forefoot. Arthrosis is less common in the midfoot, and the literature suggests it is usually posttraumatic. We found only eight references related to non-Charcot arthropathy of the midfoot1-8, two of which included patients with primary arthrosis in the midfoot2,8. However, we have treated patients with clinically relevant arthrosis in the tarsometatarsal joints with no history of trauma or inflammatory arthritis. We observed that the patients seemed to have an abnormally high ratio of the second metatarsal to the first metatarsal length. Because this anatomy had an effect on how the surgery was performed, we wanted to explore whether the observation was significant and whether metatarsal length might be associated with primary midfoot arthrosis.

Morton9 initially described an association between a short, and often hypermobile, first ray and midfoot and forefoot dysfunction. He postulated that the second metatarsal and the second tarsometatarsal joint are subjected to increased stress during weight-bearing secondary to the relative shortness and/or instability of the first ray. The relatively short first metatarsal unloads its ground contact points through the sesamoids and shifts weight-bearing forces laterally. He theorized that a functionally short first metatarsal or a long second metatarsal would predispose a patient to the development of arthrosis in the tarsometatarsal joints by placing abnormally large forces across this complex, secondary to the longer lever arm of the second metatarsal or the added stress from the lack of functional support from the first ray. On the basis of this assumption, we postulated that patients with midfoot arthrosis would have a greater ratio of the second metatarsal to the first metatarsal length than would similarly aged controls without midfoot arthrosis.

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Materials and Methods

We reviewed the treatment records for patients who presented for treatment of first, second, and third tarsometatarsal arthrosis over a three-year period at a tertiary university referral center. We excluded patients with a history of inflammatory arthritis, trauma, or Charcot arthropathy. Nine patients with fifteen feet met the entry criteria. There were seven women and two men with an average age of 64.2 years (range, fifty-one to seventy-five years). We also evaluated a control group of fourteen individuals (twenty-six feet) that consisted of eight women and six men with an average age of 53.6 years (range, forty-one to seventy years). Ten control subjects (twenty feet) were volunteers from another study with no history of foot pain or injury. The other four control subjects (six feet) had been treated for a traumatic injury of the hindfoot (an isolated calcaneal or talar fracture) with no history of foot pain or arthrosis; the uninjured foot of all four patients as well as the injured foot of two, which was seen to have no forefoot injury on a full-weight-bearing radiograph, were analyzed. Patients with midfoot and forefoot injury were excluded.

All subjects, both study and control, had weight-bearing anteroposterior and lateral radiographs made of the feet at one location with the same equipment. Measurements were made with use of a single ruler and goniometer. First, second, and fourth metatarsal lengths were measured on anteroposterior radiographs as described in Figure 1. The intermetatarsal angle was measured by extending the drawn axes of the first and second metatarsals until they intersected (Fig. 2).

We also measured the functional length of the first metatarsal relative to the second metatarsal axis and the functional length of the second metatarsal. These additional measurements were performed to eliminate the bias that might have been introduced by a wide intermetatarsal angle, which could create a functionally short first metatarsal. A line perpendicular to the second metatarsal axis was drawn tangential to the distal articular surface of the first metatarsal. The functional length of the first metatarsal was then measured on the axis line of the second metatarsal (the distance from the proximal articular surface of the second metatarsal to the perpendicular line at B in Fig. 3). The functional length of the second metatarsal was defined as the distance from the distal articular surface of the first metatarsal to the distal articular surface of the second metatarsal as measured on the second metatarsal axis (the distance from A to B in Fig. 3). The second measurement determined the functional difference between the first and second metatarsal lengths. In essence, it is the distance by which the second metatarsal exceeds the functional length of the first, and it was used to determine a functional length ratio. We assumed that the second metatarsal represents the longitudinal axis of the foot and that the farther from it that the first metatarsal drifts, the less weight-bearing function it has. These actual and functional metatarsal length ratios were used to account for differences in the sizes of the individuals and potential differences in radiographic magnification.

To identify whether the second metatarsal was long or the first metatarsal was short, we compared the lengths of both with the length of the fourth metatarsal. The fourth metatarsal was chosen because it was easily measured and could serve as an internal control.

First, second, and fourth metatarsal lengths also were measured in five of the control subjects (ten feet) on computed tomography scans in order to test the accuracy of the radiographic measurements. Spiral computed tomography scans were performed with 1.5-mm cuts. The computed tomography data were developed into three-dimensional models with use of NIH Image, which is a public domain program developed at the National Institutes of Health. (It can be downloaded at rsb.info.nih.gov/nih-image/download.html.) The metatarsal length measurements were made with use of form-Z (AutoDesSys, Columbus, Ohio). Each metatarsal was aligned on an anteroposterior radiograph and measured from end to end along a line bisecting the bone. The length of the bones as measured on the computed tomography scans was compared with the length as measured on plain radiographs.

Statistical comparisons between the study and control groups were carried out with Student (two-tailed) t tests.

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Results

On the weight-bearing anteroposterior radiographs, the average length (and standard deviation) of the first metatarsal was 64.3 ± 4.4 mm in the study group and 66.8 ± 5.9 mm in the control group. The average length of the second metatarsal was 83.7 ± 6.9 mm in the study group and 81.5 ± 6.9 mm in the control group. The average length of the fourth metatarsal was 75.5 ± 6.3 mm in the study group and 75.3 ± 6.0 mm in the control group. There were no significant differences between the two groups with regard to the first, second, or fourth metatarsal lengths. Radiographic data on all of the subjects are shown in the Appendix.

When the metatarsal lengths were measured on the computed tomography scans of five of the control subjects, the average length of the first metatarsal was found to be 63.4 ± 5.5 mm on the scans compared with 64.1 ± 5.6 mm on the plain radiographs, the length of the second metatarsal was found to be 76.2 ± 6.2 mm on the scans compared with 77.8 ± 6.5 mm on the plain radiographs, and the length of the fourth metatarsal was found to be 68.3 ± 4.6 mm on the scans compared with 71.6 ± 5.3 mm on the plain radiographs. No significant differences between the computed tomography and plain radiographic measurements were found for the first and second metatarsal lengths, but a significant difference was found for the fourth metatarsal length (p < 0.0006). This may be because the fourth metatarsal is not in the plane of an anteroposterior radiograph. The difference in the length of the fourth metatarsal affected one ratio, the ratio of the first to the fourth metatarsal length (p < 0.0344). There were no significant differences between the computed tomography and radiographic measurements for any of the other metatarsal lengths or ratios. The average difference between the computed tomography and radiographic measurements was 1.9 ± 2.5 mm (range, 0.3 to 5.4 mm), or about 3%.

The actual and functional first to second metatarsal length ratios were significantly different between the study and control groups (p < 0.0004 and p < 0.0001, respectively). In the study group the first metatarsal length was, on the average, 77.0% (95% confidence interval, 75.2% to 78.7%) of the second metatarsal length, whereas in the control group the first metatarsal length was an average of 82.0% (95% confidence interval, 80.2% to 83.9%) of the second metatarsal length. The functional second metatarsal length (Fig. 4) was, on the average, 18.6% (95% confidence interval, 13.9% to 23.3%) greater than the functional first metatarsal length in the study group and only an average of 4.1% (95% confidence interval, 1.6% to 6.9%) greater than the functional first metatarsal length in the control group. The functional second metatarsal length was significantly greater (p < 0.0001) in the study group (average, 11.9 ± 5.1 mm) than in the control group (average, 2.6 ± 4.4 mm). The difference between the two groups was approximately 9 mm.

In an effort to determine whether the first metatarsal was short or the second metatarsal was long, we compared the study and control groups with regard to the ratio of the first to the fourth metatarsal length and the ratio of the second to the fourth metatarsal length. The average length of the first metatarsal relative to the fourth metatarsal (Fig. 5) was significantly less (p < 0.0016) in the study group (average, 85.2% [95% confidence interval, 83.4% to 87.0%]) than in the control group (average, 88.6% [95% confidence interval, 87.4% to 89.9%]). The average length of the second metatarsal relative to the fourth metatarsal (Fig. 6) was also significantly greater (p < 0.0346) in the study group (average, 110.8% [95% confidence interval, 109.0% to 112.7%]) than in the control group (average, 108.2% [95% confidence interval, 106.7% to 109.8%]). Thus, patients with midfoot arthrosis had both a long second metatarsal and a short first metatarsal compared with a control group without arthrosis of the tarsometatarsal joints.

The first-second intermetatarsal angle was not significantly different between the study group (average, 9.5° ± 3.1°) and the control group (average, 8.4° ± 2.6°).

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Discussion

This study suggests an association between an anatomic variant, a high second metatarsal to first metatarsal length ratio, and an uncommon disease. Midfoot arthrosis is so uncommon that most clinicians never have the opportunity to observe this association. How this anatomic variation relates to primary arthrosis of the midfoot is unclear, but a short first metatarsal or a long second metatarsal is associated with midfoot arthritis.

Primary arthrosis of the midfoot has not been discussed much in the literature, and etiological factors have not been identified, to our knowledge. It has been suggested9 that patients with a functionally short first ray (or a functionally long second ray) have radiographic evidence of overload stresses on the second metatarsal segment, consisting of thickening of the cortices and/or shaft or a gap between the first and second cuneiforms. Morton9 noted that the second tarsometatarsal joint was not built to bear this extra stress, as its vertical depth is half that of the first tarsometatarsal joint and its morphology is substantially dissimilar. Early in the disease process, pain at the so-called Morton's point9 may be the first evidence that the second tarsometatarsal joint is being overloaded and that synovitis is developing. The increased stresses on the second metatarsal and second tarsometatarsal joint may accumulate over time and result in arthrosis.

A shortcoming of this study is its small sample size, which was due to the uncommon nature of the problem. The patients were entered into the study over a three-year period. The restrictive nature of patient privacy regulations blocked our access to patients prior to that time period, as prior approval from those patients had not been obtained. Another shortcoming of the study is the limitation of imaging in a retrospective study. While the technique was standardized, the technicians and patients were not. We tried to minimize variations due to radiographic technique by using the same x-ray machine in the same manner for all subjects. Also, the analyzed data were all relative measurements with internal controls.

Another potential area of concern was whether a two-dimensional measurement might be inadequate for a three-dimensional structure. A metatarsal that is more sharply angled relative to the floor might measure shorter than one parallel to the floor on plain weight-bearing radiographs. This concern was addressed by comparing plain radiographic measurements with those made on computed tomography scans, and we found no significant difference in the length of the first or second metatarsal between the radiographic and computed tomography measurements of the same foot. In addition, ratios of the lengths measured on the computed tomography scans did not differ from the same ratios measured on plain radiographs. The computed tomography measurements of the fourth metatarsal, however, appeared to differ from the lengths measured on the radiographs. This slight difference might be explained by the fact that the fourth metatarsal was more difficult to measure than the first three metatarsals on plain radiographs as it is on the downward slope of the transverse metatarsal arch. Its proximal end is partially obscured and out of plane on the anteroposterior radiograph. It might be more accurately measured on an oblique radiograph. However, the first and second metatarsals were the focus of this study, and they are not easily measured on an oblique radiograph.

A widened first-second intermetatarsal angle would theoretically affect the measurement of the second metatarsal length. We addressed this issue by measuring the functional length, which we defined as the length of the first metatarsal as projected along the axis of the second metatarsal. This measurement showed the same results as the actual length measures. In addition, there was no significant difference between the two groups with regard to the intermetatarsal angle, so the groups can be compared without using the functional lengths.

The patients in our study group were an average of 64.2 years of age and had no history of trauma or inflammatory arthritis. The functional length of the second metatarsal in those patients was approximately 9 mm longer than that in the control group. Also, in comparison with the fourth metatarsal, the first metatarsal was short and the second metatarsal was long. While this study does not prove that the functionally long second metatarsal predisposes the midfoot to arthrosis, it does demonstrate a clear association between midfoot arthrosis and a functionally long second metatarsal and suggests a possible mechanical etiology for midfoot arthrosis. We cannot make concrete suggestions for prevention of the disease process, but the first step in developing prevention strategies is to identify risk factors involved in the disease.

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Appendix

Specific data from the radiographs of all subjects are available with the electronic versions of this article, on our web site at jbjs.org (go to the article citation and click on “Supplementary Material”) and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM). ▪

The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at the Department of Orthopaedics and Sports Medicine, University of Washington, Harborview Medical Center, Seattle, Washington

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