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Comparisons of “unlike” exemplars are sometimes employed when evaluating a shod foot impression with a non-shod barefoot impression. The validity of that method is questioned because shoes do not always match the foot in regard to fit and because shoes of different widths and lengths can affect the dynamic anatomical positioning of pedal soft tissue and osseous structures and pressure weight distribution.

Introduction
Shoeprints left at the scene of a crime may be from a shoe that may be fitted properly to the wearer only 25–33% of the time. [1,2,3,4] That means that in the majority of cases, the shoes will not match the foot in regard to fit. In addition, 34.9% of the participants in a study by Schwarzkopf which researched the mismatch of foot and shoe size in three different populations, were found to be wearing shoes within a full two size difference and 11.9% were wearing shoes within a full three size difference. [6,7]

This means that the analysis, comparison and evaluation of the wear features and patterns of the insole/sock liner are necessary to determine pedal identifying characteristics of the wearer. “Although it is generally recommended in forensic examinations to compare “like to like” exemplars, such as a shoe insole impression to another shoe insole impression, occasionally it is necessary to compare an inked barefoot impression to an impression inside a shoe.” [8] A preliminary study by Kagan of static foot impressions revealed that since shoes do not always match the foot in regard to fit due to irregular fit of different lengths and widths, variations of the anatomical positioning of pedal soft tissue and osseous structures result. [9]

It is important to be aware that 90% of people wear shoes narrower than their feet. [10] Females generally wear shoes narrower and shorter than males. The result of wearing of shoes narrower than the feet is generally associated with corns, hallux valgus deformation, and foot pain. Snug or narrow fit has a negative effect on gait because the natural expansion of the foot with each weight-bearing step is prevented. Transverse plane spreading of the metatarsus (splaying of the forefoot) is evidenced by a gradual increase in the width of the forefoot relative to the rest of the foot. Abnormally large inter-metatarsal angulations develop between all the metatarsals however the primary spreading (splaying) occurs between the 1st and 2nd and between the 4th and 5th metatarsals. Splaying is a progressive acquired deformity that may occur rapidly over the first two to three decades of life or may progress slowly throughout an individual’s active locomotive life. [11] The normal plantar surface at the ball of the feet (metatarsal head region) is diminished [12] when wearing a shoe narrower than the foot and would not reveal that degree of splaying if compared to a barefoot impression.

When barefoot, the digits rest flat, the tips grasping the ground and assisting propulsion. Inside a shoe, that grasping function is decreased as the digits are lifted off the ground. They are functionally denied much or all of their ground grasping action and exercise so essential to exercising of the whole foot because 18 of the foot’s 19 tendons are attached to the toes. [12] The muscle with the preponderance of power would determine the direction in which the proximal phalanx would be displaced upon its respective metatarsal head. Any extensive imbalance of power between the two interossei, which insert into each side of the proximal phalanx would render the proximal phalanx unstable at the metatarsophalangeal joint. Consequently the proximal phalanx would buckle into dorsiflexion. [11] In the initial stages of pedal deformities, the deformity itself may be flexible meaning that the shod foot may reveal different degrees of deformity than the unshod foot where the foot is able to extend unrestricted. This has to be taken into account when evaluating insole/sock liners retrieved from shoes at a crime scene.

The purpose of the static foot impression study by Kagan [9] was to raise an awareness of the pedal soft tissue and osseous positional changes when wearing shoes of different lengths and widths. How much more so in a dynamic ambulatory footprint impression. 

Research Method
An initial dynamic study of ambulatory foot impressions was undertaken with shoes of the same brand (Timberland®) and model but of different lengths and widths (8M, 9W, 10M, 11M) to compare with an ambulatory barefoot foot impression to determine the degree and extent of any change. [This study was undertaken with Sally Crawford, MS Biomedical Engineering, Gait and Movement Specialist at NORAXON]. The subject was a 58 year old male with a height of 69inches, and weight of 185lbs. who was recorded wearing each shoe at least 50–70 feet in a straight line using the Medilogic® wireless insole pressure measuring system which records static/dynamic pressure of the plantar foot in shoes with thin flexible imbedded sensor insoles. The FDM(Force-Distribution-Measurement) S/SX Multifunction Force Measuring Plate® was also employed. The platform consists of a multitude of individually calibrated sensors that allow an analysis of the distribution of static/dynamic forces under the foot both walking/standing.

Using the Bare Footprint Outline as our standard, we initially compared the dynamic Barefoot platform report. [Figure 1]

Figure 1: Comparison of foot outline with dynamic barefoot impression.

As we can see, both fifth toes are visually absent on the barefoot foot impression. There is a diminished mid-foot lateral arch line. There is also a noted variation of plantar pressure points on the barefoot. On the left foot, the metatarsal phalangeal joint areas 1 and 2 recorded the maximum pressures. On the right foot the maximum pressure was recorded at the 1,2,3, and 5 metatarsal phalangeal joint areas. It should be also noted that the patient was three month status post surgical repair of a right Achilles tendon rupture.   

Figure 2: Comparison of foot impression in 8M and 9W shoes.

In Figure 2 the same dynamic foot functioning in 8M and 9W shoes was compared in regard to weight distribution of the digits, the arch, the forefoot, and the heel. There is greater definition of digital impressions and the beginning of a web space outline in the left shoe 9W, as well as greater weight distribution in the forefoot, a more consistent lateral arch and a more defined equalized heel weight distribution. The foot was contracting in the shorter 8M shoe and was able to extend itself more in the longer 9W shoe. We can also see an extension of the right foot in the 9W shoe with greater weight bearing despite the more contracted foot s/p Achilles surgical repair.

Figure 3: Comparison of foot impression in 10M and 11M shoes.

In Figure 3 when the same dynamic foot functioning in 10M and 11M shoes was compared, it was noted that there was greater digital pressure distribution in the 10M shoe. Web space digital islands were noted in the left 10M shoe but not in the 11M shoe. There was greater weight bearing pressure at the left 1st and 5th metatarso-phalangeal joints in the 10M shoe and there was greater lateral arch pressure distribution in the left 10M shoe.

Figure 4: Comparison of foot impression in 8M and 11M shoes.

In Figure 4 when comparing the same foot in 8M and 11M shoes one can see the variations in plantar weight distribution in the digital areas, the web space outline, the web ridge line, the arch line and the heel.

Figure 5: Comparison of foot impression barefoot and in 8M shoe.

Comparing a barefoot dynamic foot impression with the same foot in size 8M shoe shows that the defining features between the two foot impressions are non-similar. The 8M shoe foot impression fails to exhibit the digital impressions and maximal metatarsal pahalangeal joint pressure impressions noted in the barefoot. The heel pressure distribution is off center medially in the 8M shoe compared with the barefoot impression.

Figure 6: Comparison of foot impression barefoot and in 10M shoe.

Comparing a barefoot dynamic foot impression with the same foot in size 10M shoe shows that digital impressions are similar on the left foot. The heel pressure is centered on the left and the maximal metatarsal phalangeal joint pressure is more consistent on the left even though there are still variations. On the right, there is similarity with slight variations of digital impressions, the metatarsal phalangeal joint maximal pressure is similar but there is still off center medial maximal heel pressure. 

Variations must be considered when examining like and “unlike” exemplars. “Although it is generally recommended in forensic examination to compare “like to like” exemplars, such as a shoe insole impression to another shoe insole impression, occasionally it is necessary to compare an inked barefoot impression to an impression inside a shoe.” [8]

Conclusion
It is difficult to draw conclusions from a study of a single subject. However the purpose of this paper was to raise awareness that there are plantar pressure distribution changes due to soft tissue and osseous positional changes when wearing shoes of different sizes and these must be considered when comparing shoe insole foot impressions with barefoot impressions and must also be considered when comparing shoe insole foot impressions.

These variations must be researched in greater depth in a larger study to promote a more valid forensic examination of dynamic barefoot and shod foot impressions. A secondary question arises in that if there are changes in foot print impressions wearing different sized shoes, will wearing different sized shoes affect the gait pattern. Further studies are necessary to evaluate.

Acknowledgement
Special appreciation to Sally Crawford, MS Biomedical Engineering, Gait & Movement Performance Specialist, NORAXON® 15770 N.Greenway-Hayden, Scottsdale, Arizona 85260

Special thanks to S & H Uniforms, 1 Aqueduct Road, White Plains, New York 10606, for their assistance in providing the shoes in this study.

Dr.Bryan B. Kagan, DPM is a board certified podiatrist in private practice since 1980 in White Plains, New York. He specializes in Forensic Podiatry and Podiatric Medicine and is on staff at White Plains Hospital, White Plains, New York. He trained in forensic podiatry under Dr. John DiMaggio, the founder of the American Society of Forensic Podiatry, and has been active in forensic research and consulting since 2008. White Plains Center for Foot Care; 122 West Post Road; White Plains, NY 10606; westfootdoc@gmail.com; http://www.bkagan-dpm-forensicpodiatry.com

References

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