The document summarizes research on using biological adjuncts to enhance flexor tendon healing. It discusses how surgical techniques and rehabilitation have improved results but adhesion formation remains a challenge. Various adjuncts are explored including growth factors, platelet rich plasma, stem cells, and gene therapy which show potential to aid faster healing with less adhesions. Ultrasound, magnetic fields, and rhynchophylline have also demonstrated positive impacts on tendon repair in animal studies. Continued research seeks safer and more effective ways to biologically manipulate the healing process at the cellular level.

1. The search for biological adjuncts
to enhance flexor tendon healing
Alphonsus Chong
Associate Professor, Department of Orthopaedic Surgery, NUS
Group Chief, Hand and Reconstructive Microsurgery, NUHS
No relevant conflicts of interest to disclose

2. Flexor tendon results have improved greatly
• Flexor tendon repair, rehabilitation and
results have come a long way
• Better surgical technique and rehabilitation
with scientific basis and clinical results
• Still advancing:
• WALANT
• Pulley venting
• Evolving repair techniques
• Early active motion rehabilitation (Neiduski 2019)
Tang, Jin Bo. “New Developments Are Improving Flexor Tendon Repair:” Plastic and Reconstructive
Surgery 141, no. 6 (June 2018): 1427–37. https://doi.org/10.1097/PRS.0000000000004416.

3. Why adjuncts to tendon healing?
“Despite more recent minor alterations in suturing technique, uniformly
excellent results are still not achievable. Perhaps the next significant jump in
improved results lies in our understanding and manipulation of wound
healing at the cellular level and elimination of adhesions formation in the
repaired flexor tendon and digital sheath” (Kleinert 1995)
Kleinert, H. E., Špokevičius, S., & Papas, N. H. (1995). History of flexor tendon repair. The
Journal of Hand Surgery, 20(3), S46–S52. https://doi.org/10.1016/S0363-5023(95)80169-3

4. How can adjuncts to tendon repair help?
•Faster healing
•Less adhesions
•‘better’ healing
• Tendon loss
• tendinopathy
•Immobilization - ensures
repair integrity, but increases
adhesions
•Mobility - decreases
adhesions, increases strength,
but risks repair rupture
• Balance achieved by
protected mobilization

5. Tendon healing biology - cellular events
• Phases (Gelbermann)
• Inflammatory
• Fibroblastic
• Remodelling
• Extrinsic and intrinsic (Gelberman, Lundborg)
• Extrinsic Active earlier
• Predominance of extrinsic leads to scar formation
• Early motion helps intrinsic healing

6. Adhesion formation and prevention
• Injury (including surgery)
• Breaches cell basement
membrane
• sparks inflammatory response
• Interaction between
inflammatory response,
coagulation cascade, and
angiogenesis
• Hypoxia drives fibroblast
differentiation to adhesion
phenotype fibroblasts
Capella-Monsonís, H., Kearns, S., Kelly, J. et al. Battling
adhesions: from understanding to prevention. BMC biomed
eng 1, 5 (2019). https://doi.org/10.1186/s42490-019-0005-0

7. Non-biological adjuncts
• Biophysical modalities
• Ultrasound
• Early post repair increases range of movement, advanced scar
maturation and decreased inflammation around repair (Gan 1995)
• Magnetic Fields (see Henry 2008)
• Chemical
• Mannose-6-Phosphate
• Vitamin C (LK Hung)
• Rhynchophylline (QQ Yang)
• NSAIDs
• Physical barriers – to prevent adhesions
• Seprafilm – used in abdominal surgery
• Amnion
• Other barriers

8. Biological adjuncts
• Growth factors
• TGFß, PDGFs and others
• Combination
• Platelet rich plasma (PRP): autologous,
simple preparation and delivery
• Many biologically active factors – many
positively influencing tendon healing, but
some negative ones as well
• Cells
• Stem cells e.g.
• Mesenchymal stem cells
• Other cells e.g. tenocytes
• Gene therapy
https://www.researchgate.net/publication/331116076_Mechanisms_of_Action_
of_Multipotent_Mesenchymal_Stromal_Cells_in_Tendon_Disease/figures?lo=1

9. Growth factor supplementation
• Single growth factors
• TGF-Beta and its pathways
• PDGF
• bFGF
• IGF
• VEGF (see Kollitz)
• Combination of growth factors
• IGF, PDGF, bFGF
• Limitations
• Dosing, availability and persistent at
the repair site
Costa, M. A., Wu, C., Pham, B. V., Chong, A. K. S., Pham, H. M., & Chang, J.
(2006). Tissue engineering of flexor tendons: Optimization of tenocyte
proliferation using growth factor supplementation. Tissue Engineering, 12(7),
1937–1943. https://doi.org/10.1089/ten.2006.12.1937

10. Platelet rich plasma
• Platelets, bioactive factors (e.g. TGF-Beta, HGF), plasma, and some cells
• Platelets contain large amounts of GFs
• Pros: Autologous, simple to prepare and deliver
• Cons: variable in contents (see Dohan 2009)
• L-PRP vs P-PRP: leukocytes negatively affect tendon healing (e.g. increase
inflammation)
• PRPLP vs PRPHP
• Many in vitro studies show positive effects on tendon proliferation,
differentiation, anabolic effects and decreased inflammation
• In-vivo studies some mixed results

11. Platelet rich fibrin on rabbit
flexor tendon healing
• Platelet rich plasma – autologous
but uses bovine thrombin
• Platelet rich fibrin – completely
autologous
• Fibrin network – cell migration,
controlled release of growth factors
• Applications in dental surgery
• No increase in AROM, lower load
and stress to failure
• Other studies
• No benefit (Kollitz 2014)
• Limitations: dosing, half-life, competing
and compensatory mechanisms
Liao, J. C. Y., He, M., Gan, A. W. T., & Chong, A. K. S. (2017). The Effects of Autologous Platelet-Rich Fibrin
on Flexor Tendon Healing in a Rabbit Model. The Journal of Hand Surgery, 42(11), 928.e1-928.e7.
https://doi.org/10.1016/j.jhsa.2017.06.098

12. Cell therapy
• Different cell sources
• Tenocytes
• Limited
• Need separate procedure for harvest
and expansion
• Stem cells
• MSCs - supply from fat/ bone marrow
• Autologous still needs a separate
procedure
• vs allogeneic
• But other concerns like safety and
immune response
https://www.researchgate.net/publication/331116076_Mechanisms_of_Action_
of_Multipotent_Mesenchymal_Stromal_Cells_in_Tendon_Disease/figures?lo=1

13. Allogeneic bMSCs positively influence tendon healing
• Rabbit Achilles tendon primary
repair
• Randomized to bMSCs or control
• bMSC group
• Cells remain viable up to 6
weeks
• At 3/52, better:
• morphometric parameters
• material properties
• Same at 6/52 and 12/52
• “faster” but not “finally stronger”
Chong, A. K. S., Ang, A. D., Goh, J. C. H., Hui, J. H. P., Lim, A. Y. T., Lee, E. H., & Lim, B. H. (2007). Bone
marrow-derived mesenchymal stem cells influence early tendon-healing in a rabbit achilles tendon
model. J Bone Joint Surg Am, 89(1), 74–81. https://doi.org/10.2106/JBJS.E.01396
Viable cells
1 week
3 weeks
6 weeks
Better material properties
Better nuclear morphometric parameters
at 3 weeks in bMSC group
Nuclear Aspect Ratio = b/a
a b
Nuclear Orientation Angle

14. Gene therapy
• 2021: inflection point – beginning
to see therapeutics reach regular
clinical use after decades of
research
• Possible sequence of adoption –
see below
• In tendon healing
• VEGF (Mao 2017), bFGF (Tang 2008)
• Overcomes half-life problem
Mao, W., Wu, Y., Yang, Q. et al. Modulation of digital flexor tendon healing by vascular endothelial growth
factor gene transfection in a chicken model. Gene Ther 24, 234–240 (2017).
https://doi.org/10.1038/gt.2017.12
VEGF: smoother Stronger

15. Summary
Improving
flexor
tendon
healing
Surgical &
Repair
techniques
Rehabilitation
protocols
Biological
adjuncts
Basic, translational, and clinical knowledge and expertise
Growth factors
Cells: various stem cells
Gene therapy
The search continues!

16. Additional References
1. Dohan Ehrenfest, D. M., Rasmusson, L., & Albrektsson, T. (2009). Classification of platelet concentrates: From pure platelet-rich plasma (P-
PRP) to leucocyte- and platelet-rich fibrin (L-PRF). Trends in Biotechnology, 27(3), 158–167. https://doi.org/10.1016/j.tibtech.2008.11.009
2. Gan, B. S., Huys, S., Sherebrin, M. H., & Scilley, C. G. (1995). The Effects of Ultrasound Treatment on Flexor Tendon Healing in the Chicken
Limb. Journal of Hand Surgery, 20(6), 809–814. https://doi.org/10.1016/S0266-7681(95)80054-9
3. Henry, S. L., Concannon, M. J., & Yee, G. J. (2008). The effect of magnetic fields on wound healing: Experimental study and review of the
literature. Eplasty, 8, e40.
4. Kollitz, K. M., Parsons, E. M., Weaver, M. S., & Huang, J. I. (2014). Platelet-Rich Plasma for Zone II Flexor Tendon Repair. HAND, 9(2), 217–
224. https://doi.org/10.1007/s11552-013-9583-9
5. Leong, N. L., Kator, J. L., Clemens, T. L., James, A., Enamoto‐Iwamoto, M., & Jiang, J. (2020). Tendon and Ligament Healing and Current
Approaches to Tendon and Ligament Regeneration. Journal of Orthopaedic Research, 38(1), 7–12. https://doi.org/10.1002/jor.24475
6. Neiduski, R. L., & Powell, R. K. (2019). Flexor tendon rehabilitation in the 21st century: A systematic review. Journal of Hand Therapy,
32(2), 165–174. https://doi.org/10.1016/j.jht.2018.06.001
7. Singh, R., Rymer, B., Theobald, P., & Thomas, P. B. M. (2015). A review of current concepts in flexor tendon repair: Physiology,
biomechanics, surgical technique and rehabilitation. Orthopedic Reviews, 7(4). https://doi.org/10.4081/or.2015.6125

17. Supplementary Slides

18. History
• 980-1037 AD: Avicenna – direct tendon repair  forgotten (Kleinert)
• Subsequently repair avoided
• – Galen held it lead to convulsions: probably not differentiating tendons from
nerves
• 18-19th Century: von Haller, Syme, Nicoladoni and others described
repair techniques
• Bunnell 1940s:
• Tendon grafting instead of primary repair
• Kleinert 1950s-70s – primary repair can give good results

19. Assessment of outcomes
• Strickland
• Buck-Gramcko
• TAM
• ASSH
• The scores do not correlate well. In
extreme examples, the same AROM
can be classified as poor or
excellent depending on the score
used.
• Present the actual AROM rather
than score used
Hahn, P., Kirchberger,M. C., Unglaub, F., & Spies, C. K. (2016).[How Congruent is the Rating of the Resultsof Flexor Tendon Injury Repairs Using the Scores by Buck-Gramcko,Strickland, and the American
Society for Surgery of the Hand?].Handchirurgie,Mikrochirurgie,PlastischeChirurgie:Organ Der DeutschsprachigenArbeitsgemeinschaftFur Handchirurgie: OrganDer Deutschsprachigen
ArbeitsgemeinschaftFur MikrochirurgieDer PeripherenNerven Und Gefasse:Organ Der V...,48(5), 290–295. https://doi.org/10.1055/s-0042-111892

20. The use of outcome scores for flexor tendon
• Various methods of grading motion  qualitative outcomes
• Quantitative physician assessments: ASSH, Strickland, Strickland-Glogovac –
does not reflect patient outcomes (VAS, DASH)
• DIPJ AROM and Total AROM correlate better with patient perceived outcomes
(VAS and DASH)
• Variation in practice seen
• UK study
Karjalainen, T., Jokinen, K., Sebastin, S. J., Luokkala, T., Kangasniemi, O.-P., & Reito, A. (2019). Correlations Among
Objectively Measured Impairment, Outcome Classification Systems, and Subjectively Perceived Disability After Flexor
Tendon Repair. The Journal of Hand Surgery, 44(5), 361–365. https://doi.org/10.1016/j.jhsa.2018.06.010

21. Biologics for tendon repair.
• Splint evolution
• No splint
• Protocols

22. Biophysical stimulation
• Ultrasound
• Early post repair increases range of movement, advanced scar maturation and
decreased inflammation around repair (Gan 1995)
• Magnetic Fields (see Henry 2008)
• Prevent adhesions
• Wrap?
• Application?
• PRP

23. Low Intensity Pulsed Ultrasound (LIPUS)
• Low intensity avoids thermal
effects
• Acoustic cavitation, biological
signalling
• LIPUS: 1-3 MHz, 0.02-1 W/cm2
SATA (Spatial average,
Temporal average) @ 5-20
mins a day
Jiang, X., Savchenko, O., Li, Y., Qi, S., Yang, T., Zhang, W., & Chen, J. (2019). A Review of Low-Intensity Pulsed Ultrasound for
Therapeutic Applications. IEEE Transactions on Biomedical Engineering, 66(10), 2704–2718.
https://doi.org/10.1109/TBME.2018.2889669

24. Magnetic Field and tendon healing
• Some evidence it can help bone and wound healing
• Early evidence it can help tendon healing (see Henry 2008)
• Pulsed magnetic field of low amplitude: increases tensile strength in rat
Achilles tendon
• But also conflicting information from other studies
• Mechanisms unclear
• Vascular effect
• Best protocol unclear
• Static vs PEMF
• Strength: 23 Gauss to 400 Gauss
Henry, S. L., Concannon, M. J., & Yee, G. J. (2008). The effect
of magnetic fields on wound healing: Experimental study and
review of the literature. Eplasty, 8, e40

25. Biophysical considerations
• Article focuses on PEMF (time-varying low frequency EMF)
• Used for:
• Adjuvant for MSK injuries
• Neurological conditions
• Cancer – prevents angiogenesis
• ‘biophysical dosimetry’ needed for different conditions likely to be different
• “Biological windows” concept
• Also consideration of waveforms:
• Sine vs semi-sine
• Pulsed signal
• Target – cell memberane – signal transduction
• Calcium ion Markov, M. S. (2007). Pulsed electromagnetic field therapy
history, state of the art and future. The Environmentalist,
27(4), 465–475. https://doi.org/10.1007/s10669-007-9128-2

26. Rhynchophylline improves flexor tendon
repair in chickens
• Tetracyclic oxindole alkaloid
• Active ingredient in Uncaria/ Cat’s claw/ Gambier
• Investigated as candidate drug for CVS/CNS diseases
• Previously shown to prevent intra-abdominal
adhesions
• In Flexor tendons to:
• Inhibited adhesion (via adhesion score)
• Tendon healing
• Gliding excursion
Yang, Q. Q., Zhang, L., Ju, F., & Zhou, Y. L. (2021). Sustained-Release Hydrogel-Based Rhynchophylline Delivery System
Improved Injured Tendon Repair. Colloids and Surfaces B: Biointerfaces, 111876.
https://doi.org/10.1016/j.colsurfb.2021.111876

27. Different types of PRP
• Choukroun’s PRF as the
simplest and open access
technique
Dohan Ehrenfest, D. M., Rasmusson, L., & Albrektsson, T. (2009). Classification of platelet concentrates: From pure
platelet-rich plasma (P-PRP) to leucocyte- and platelet-rich fibrin (L-PRF). Trends in Biotechnology, 27(3), 158–167.
https://doi.org/10.1016/j.tibtech.2008.11.009

28. PRF: second generation platelet
concentrate
• Issues with commercial fibrin
glues - safety
• Autologous fibrin prep
• Complex procedures
• Amount?
• PRF
• Must be fast to centrifuge – else
non-usable product
• Structure different
• ? Contains more bioactive
substances?
Fig. 4. Theoretical computer modelling of condensed tetramolecular
or bilateral fibrin branch junctions. Note the rigidity of this architecture
(D-TEP v1.3).
Fig. 5. Theoretical computer modelling of trimolecular or equilateral
fibrin branch junctions. Note the flexibility of this net architecture (D-
TEP v1.3).
Dohan, D. M., Choukroun, J., Diss, A., Dohan, S. L., Dohan, A. J. J., Mouhyi, J., & Gogly, B. (2006). Platelet-rich
fibrin (PRF): A second-generation platelet concentrate. Part I: Technological concepts and evolution. Oral
Surgery, Oral Medicine, Oral Pathology, Oral Radiology, and Endodontology, 101(3), e37–e44.
https://doi.org/10.1016/j.tripleo.2005.07.008

29. Mesenchymal stem cells increases tendon healing rate in rabbits
after primary repair
NZW Rabbits (female, 13wks)
bMSC culture
bm aspiration
allogeneic
ex-vivo expansion
Histology Biomechanics
Harvest @ 3, 6 & 12 weeks
foot
Sharp division at mid-
substance of Achilles
tendon followed by
repair
foot
Fibrin only
Fibrin & bMSC
Chong AK, Ang AD, Goh JCH, Hui JHP, Lim AY, Lee EH, Lim BH, Journal of Bone and Joint Surgery, 2007

30. Results
1 week
3 weeks
6 weeks
Tendon long axis
1 2 Better nuclear morphometric parameters at 3
weeks in bMSC group
3
Nuclear Aspect Ratio = b/a
a b
Nuclear Orientation Angle