This is, to the best of our knowledge, the first randomized prospective study of the effects of tenotomy treatment on plantar pressure of the diabetic hammertoe. In this observer blinded RCT substudy, we show that tenotomies can significantly lower PPP and PTi in the toe regions of the diabetic hammertoe. In addition, the effect of tenotomies on plantar pressure was independent of the tested clinical variables (age, gender, diabetes duration, and prior history of DFU). In the original study, we did not exclude participants with semi-rigid or rigid hammertoes, this choice was done, knowing that some hold that only flexible hammertoes can be treated with tenotomies.26 The decision was made on the basis of clinical experiences that tenotomies, in our experience, may provide sufficient off-loading of the rigid hammertoe to provide relief from the pressures leading to impending and active ulcers. The findings in this study showed a tendency toward lower plantar pressure affecting the rigid hammertoes following tenotomy, but the decline did not reach statistical significance, likely due to the low numbers. For a more comprehensive discussion on the topic, we refer to the original article.26

Waaijman and Bus suggested that PPP and PTi are interdependent when performing plantar pressure measurements.32 The findings in this study indicate that the same was true in this instance. In the following, only the PPP outcomes will be discussed.

As discussed earlier, prior studies have investigated the plantar pressure associated with the diabetic hammertoe. These studies agree that hammertoes increase the plantar pressure in the metatarsal and hallux regions.9 22 33 34 To our knowledge, only one study has reported increased pressure in the lesser toes (the four lateral toes) associated with the diabetic hammertoe.9 Despite this, a direct connection between the increased pressure and risk of DFU at the specific anatomical locations of the diabetic hammertoe has not been shown. As discussed in the Introduction, Waaijman et al previously suggested that 200 kPa may be a threshold for reulceration. In this study, we found that peak and average pressures affecting the hammertoes in many cases were significantly higher than 200 kPa, before tenotomy, declining to below this threshold following tenotomy.16

As discussed earlier, most healthcare professionals that treat the diabetic foot would prefer to use non-surgical offloading before surgery. To this end, one non-randomized study has found that non-surgical offloading, in the form of toe props, significantly reduced plantar pressure associated with the diabetic hammertoe.35 However, the use of non-surgical treatment like wearable devices only reduces the pressure when the devices are worn and adherence of individuals with diabetes and foot complications is a known challenge.36 In addition, the individual has to be able to apply the toe prop correctly, which may be a challenge if complications like neuropathy and retinopathy are present. Alternatively, a healthcare professional should apply the toe prop every day, which is time and expense consuming, while a surgical procedure could address the underlying pathology permanently.

A prospective non-randomized study by Mens et al37 found that plantar pressure of diabetic hammertoes could be reduced by tenotomizing the flexor digitorum longus tendon. We found similar results when tenotomizing both flexor digitorum longus and brevis tendons. These and the previous findings from studies on tenotomy treatment of the diabetic hammertoe21 38 indicate that the increased pressure associated with the diabetic hammertoe is directly associated with the development of DFUs. In addition, it suggests that the positive effects on prevention and treatment of the tip of the toe DFU associated with tenotomy treatment is likely due to the reduction in plantar pressure in the toe regions.

The study by Mens et al37 also showed that plantar pressure in metatarsal regions was reduced following isolated flexor digitorum longus tenotomy. We did not find a reduction of PPP in the metatarsal regions following tenotomy of both longus and brevis tendons. It is difficult to argue for an underlying anatomical explanation, as the effects of the flexor digitorum longus and brevis tendons are similar at the metatarsophalangeal joint. The observed difference could be attributed to the difference in the nature of the hammertoe deformities. We did not assess the underlying pathology leading to the diabetic hammertoe and it should be noted that the pathology of the diabetic hammertoe and hammertoes in general is unclear.39–42 In addition, we did not exclude individuals with semi-rigid or rigid deformities. Mens et al did not assess the underlying pathology either, but exclude individuals with rigid hammertoe deformities.37 Their study showed a reduction in the angle of the metatarsophalangeal joints following intervention, using pre- and post-tenotomy X-ray measurements.37 The change in metatarsophalangeal joint angle following tenotomy is likely dependent on the rigidity of the hammertoe deformity. In addition, the metatarsophalangeal joint angle is thought to play a role in the magnitude of plantar pressure in the metatarsal region.22 Further studies are needed to assess the effect of tenotomies on the plantar pressure in the metatarsal regions of the diabetic foot. Future studies should include an assessment of the rigidity of the hammertoe deformity. Furthermore, the effect of tenotomizing only flexor digitorum longus versus combined flexor digitorum longus and brevis tenotomy is unclear.

Besides the tip of the toe and metatarsal head DFUs, DFUs at the dorsal aspect of the toe have been suggested to be associated with the diabetic hammertoe.21 The increased “height” of the hammertoe results in the toe pressing against the roof of the footwear, leading to an increase in the pressure. The dorsal aspect of the toe is not accessible for pressure analysis by the system used in this study. The study by Mens et al37 showed that the angles of the metatarsophalangeal, proximal and distal interphalangeal joints of the diabetic hammertoe were reduced following tenotomy. This would theoretically result in a decreased “height” of the toe and thereby a reduction in the pressures at the dorsal aspect of the toe. The change in the “height” of the toe was, however, not reported. Further studies are needed if effects of tenotomies on pressure at the dorsal aspect of the toe are to be examined.

Despite the consensus that increased plantar pressure leads to increased risk of DFUs, there is no consensus on a plantar pressure threshold that denotes increased risk of incurring a DFU.8 Some studies suggest that the threshold lies between 588 and 981 kPa, while one study suggested a threshold as low as 335 kPa and one as high as 1206 kPa.8 43 Many of the studies that report a possible threshold examined the foot as a whole and correlated the PPP at any point of the foot to a DFU at any point of the foot.8 This could be part of the reason why a consensus on PPP threshold cannot be reached across studies. The anatomy of the planta pedis and the forefoot varies greatly as does the forces that affect the different regions of the planta and forefoot both during gait and stance. In addition, and as discussed previously, plantar pressure is not a singular concept, which may further complicate the search for a common PPP threshold associated with increased risk of incurring a DFU.6

At the baseline visit, we found that the average PPP in toe regions of index toes that either presented with or developed a DFU was 443.87 kPa. Using this as a threshold for increased risk of tip of the toe DFU, we found that a tenotomy significantly reduced the number of participants with at least one index toe over the threshold. This study was neither powered nor designed to assess a PPP threshold associated with DFUs and we do not suggest that the threshold used here should be used as a general risk predictor. We do, however, suggest that finding a consensus PPP threshold associated with increased risk of developing DFUs will be difficult and should at least be determined for separate anatomical regions of the foot in future studies. In addition, we suggest that tenotomy treatment of the diabetic hammertoe could reduce the risk of incurring a DFU associated with the diabetic hammertoe as was also reported in the RCT study where the participants for this study were recruited.26

While the results align with the theory that tenotomies reduce the pressure affecting the diabetic hammertoe, and likely gives us the cause of the positive effects on ulcer healing and prevention reported in the original study,26 the results of this study are only for the barefoot. It, therefore, may be difficult to translate the findings to in-shoe measurements. It would seem intuitive that the operated toes would show similar reductions in in-shoe plantar pressure, but the observed differences between operated and “control” may not be similar as in-shoe offloading likely reduces the pressure to some degree. However, as discussed in the Introduction, in-shoe offloading may provide sufficient offloading to alleviate the pressures affecting the hammertoe in the moment, but it does not address the underlying deformity. Future studies should focus on the effects of adding tenotomies to standard offloading on in-shoe pressure at predilection sites for DFUs associated with hammertoes.21

There are limitations to this study; the participants were not blinded to allocation. Blinding the participants would only be possible by performing a sham operation on the controls, which poses its own challenges. As discussed above, it may be difficult to translate the results to in-shoe pressure measurements.

The strengths of the study are the RCT design and the blinding of the observers.

This study is, to our knowledge, the first RCT examining effects of tenotomy on the increased PPP associated with the diabetic hammertoe. It assists in understanding the causative relationship between flexor tendon tenotomy and positive effects on prevention and treatment of the tip of the toe DFU of the diabetic hammertoe. The findings indicate that the positive effects reported to be associated with tenotomy treatment on healing and prevention of tip of the toe DFUs are directly associated with the reduction in plantar pressure.