Researchers tested a new way to deliver vitamin D to help heal shoulder injuries in rabbits. They combined two approaches: giving vitamin D by mouth and applying a special vitamin D-coated sheet directly to the injured tendon during surgery. The group that received both treatments showed the strongest healing results, with better tendon strength and muscle recovery after 12 weeks compared to groups receiving only one treatment or no vitamin D. While these results are promising, this research was done in animals, so scientists will need to test whether this approach works safely and effectively in people before it can be used in human surgery.

The Quick Take

  • What they studied: Whether combining two ways of delivering vitamin D (by mouth and directly to the injury site) helps shoulder tendons heal better after surgery
  • Who participated: 64 rabbits divided into groups that received different combinations of vitamin D treatments or no vitamin D
  • Key finding: Rabbits that received both oral vitamin D and a special vitamin D sheet at the surgery site had tendons that were 36% stronger and muscles that recovered better than rabbits receiving only one treatment or neither
  • What it means for you: This suggests that combining vitamin D delivery methods might improve shoulder surgery recovery, but human studies are needed before doctors can use this approach. If you’re having shoulder surgery, talk to your doctor about vitamin D—don’t try experimental treatments on your own.

The Research Details

Scientists divided 64 rabbits into four main treatment groups. They surgically detached the rabbits’ shoulder tendons (similar to a rotator cuff tear in humans) and then repaired them. Some rabbits received vitamin D by mouth, some received a special sheet coated with vitamin D placed during surgery, some received both treatments, and some received neither. The researchers then measured how well the tendons healed at 4 weeks and 12 weeks after surgery by looking at blood vitamin D levels, examining the tissue under a microscope, and testing how strong the healed tendons were.

This type of study is called a randomized controlled trial because the rabbits were randomly assigned to different treatment groups, and some groups served as controls (receiving no special treatment) to compare against. This design helps researchers figure out whether the vitamin D treatments actually caused the improvements in healing, rather than other factors.

Testing new healing methods in animals first is important because it helps scientists understand how the treatment works and whether it’s safe before trying it in humans. This study used a realistic injury model (a detached tendon) and measured multiple aspects of healing, including strength, tissue quality, and muscle recovery. The combination approach is novel because most previous research looked at vitamin D alone, not at combining local and systemic delivery.

This study was well-designed with random assignment to groups and multiple measurement methods. However, because it was done in rabbits, the results may not translate exactly to humans—rabbit bodies heal differently than human bodies. The sample size of 64 animals is reasonable for a preclinical study. The researchers measured several important outcomes (blood levels, gene expression, tissue structure, and strength), which strengthens the findings. The study was published in a respected medical journal focused on shoulder and elbow surgery.

What the Results Show

The most important finding was that rabbits receiving both vitamin D by mouth and the vitamin D-coated sheet (Group D) had the strongest healing results. At 12 weeks after surgery, their repaired tendons could withstand 139.6 Newtons of force before breaking, compared to about 102 Newtons in the other groups—roughly 37% stronger.

The vitamin D-coated sheets successfully delivered vitamin D directly to the injury site. Rabbits that received these sheets had higher vitamin D levels in their muscles compared to rabbits that didn’t receive them. Blood vitamin D levels were highest in the group receiving oral supplementation, showing that both delivery methods worked as intended.

Muscle recovery was also better in the combination treatment group. The muscle fibers in rabbits receiving both treatments grew larger and recovered better than in other groups. This matters because shoulder injuries affect not just the tendon but also the muscles that support the shoulder.

At the 4-week mark (early healing phase), the combination treatment group showed higher levels of collagen type 1, which is the main protein that makes tendons strong. The special vitamin D sheets also successfully delivered vitamin D to the muscle tissue, as measured by laboratory tests. Rabbits receiving the vitamin D sheets showed better tissue structure under the microscope, with denser and more organized collagen fibers compared to rabbits without the sheets.

Previous research has shown that vitamin D is important for bone and tendon health, but most studies looked at vitamin D alone. This study is novel because it combines two delivery methods—systemic (whole-body) and localized (at the injury site). The results suggest that combining approaches may be better than either method alone, which is a new finding in the field. The use of 3D-printed nanofiber sheets is a modern technology that hasn’t been extensively studied for tendon healing before.

The biggest limitation is that this study was done in rabbits, not humans. Rabbits heal differently than people, and their tendons are smaller and structured differently. The study only followed healing for 12 weeks, but human tendon healing can take much longer. The vitamin D sheets are experimental technology that hasn’t been tested in humans yet, so we don’t know if they would work the same way or if they would be safe. The study didn’t test different doses of vitamin D or different sheet designs, so we don’t know the optimal approach. Finally, this was a preclinical study, meaning it’s the first step—many promising animal studies don’t lead to successful human treatments.

The Bottom Line

Based on this animal research, combining vitamin D delivery methods appears promising for tendon healing (moderate confidence level). However, these findings cannot yet be applied to human patients. Current recommendations remain: maintain adequate vitamin D levels through diet or supplementation as recommended by your doctor, follow post-surgery rehabilitation protocols prescribed by your physical therapist, and avoid experimental treatments not approved for human use. If you’re facing shoulder surgery, discuss vitamin D status with your surgeon, but don’t expect this specific nanofiber sheet technology to be available yet.

This research is most relevant to orthopedic surgeons and researchers working on tendon injuries and rotator cuff repairs. Patients with rotator cuff tears or those planning shoulder surgery should be aware of this research direction but understand it’s not yet ready for clinical use. People interested in sports medicine and injury recovery may find this interesting. This research does NOT currently apply to anyone seeking treatment—it’s a foundational study for future human research.

In this rabbit study, significant improvements were visible at 4 weeks and most pronounced at 12 weeks. If this technology eventually reaches human trials, recovery timelines would likely be longer because human tendons are larger and heal more slowly. Realistic expectations for human application would be several years away, requiring safety testing and human clinical trials first.

Want to Apply This Research?

  • Track daily vitamin D intake (from food and supplements) and shoulder strength/range of motion using simple tests like how high you can raise your arm or how much weight you can lift. Record weekly measurements to monitor progress.
  • If you’re recovering from shoulder surgery or injury, use the app to remind you to take vitamin D supplements as recommended by your doctor, log your physical therapy exercises, and track pain levels. This data can help your doctor see if vitamin D supplementation is helping your recovery.
  • Set up monthly check-ins to review vitamin D levels (through blood tests ordered by your doctor), track shoulder function improvements, and monitor muscle strength gains. Compare measurements over 3-month periods to see long-term trends in your recovery.

This research was conducted in rabbits and has not been tested in humans. The vitamin D nanofiber sheets described in this study are experimental and not approved for human use. These findings do not constitute medical advice and should not be used to guide treatment decisions. If you have a rotator cuff tear or are planning shoulder surgery, consult with your orthopedic surgeon or healthcare provider about appropriate treatment options. Do not attempt to use experimental treatments or devices without medical supervision. While maintaining adequate vitamin D levels is generally recommended for bone and muscle health, any supplementation should be discussed with your doctor, especially if you have existing health conditions or take medications.