*2.7. Postoperative Rehabilitation*

The patient was kept in a hip and knee brace locked at 70 degrees and 30 degrees of flexion, respectively, for 8 weeks, not allowing the weight-bearing walk and using two crutches (Figure 7a). After 8 weeks, we removed the hip and knee brace and the patient underwent a rehabilitation program with a passive progressive range of motion of the hip and knee, allowing partial weight-bearing using two crutches. The US performed at week 12 showed images compatible with the integration of the allograft into the tendon (Figure 7b), which was the start point of an active rehabilitation program (Figure 7c). In a gradual manner, and always supervised by the physiotherapist and surgeon advice, our patient initiated quadriceps and hamstring isometric, eccentric and proprioceptive exercises, as well as active resistance strength exercises. After 12 months, the patient resumed her active lifestyle without any limitation.

**Figure 7.** Postoperative rehabilitation. (**a**) Immobilizing splints keeping the knee and hip flexed. (**b**) US (week 12 postoperatively) showed images compatible with the integration of the allograft. (**c**) Rehabilitation process starting at 12 weeks.

#### **3. Discussion**

We describe the reconstruction of a chronic proximal hamstring tear using a semitendinosus tendon allograft sutured in a "V inversion" manner and assisted with PRGF as a novel surgical technique to treat a chronic proximal hamstring tear. Our patient resumed her active lifestyle without any limitation 12 months after the surgery. This new technique offers mechanical and biological advantages to tackle the large retraction of hamstring stumps and the entrapment of the sciatic nerve within the scar. In fact, 9 months after the surgery, the patient resumed her previous lifestyle, including recreational sport.

There have been reported numerous different procedures to overcome and bridge tear gaps superior to 5 cm between the tendon stumps and the ischial tuberosity in chronic proximal hamstring tears by using ipsilateral distal hamstring autografts [8] or Achilles tendon allografts [9], both with good post-operative clinical outcomes and patient satisfaction with 24 and 48-month (long term) follow-ups. Despite some inherent potential drawback of allografts, including infection and disease transmission, issues with the osseointegration and the cost and shortage of allografts, the surgical repair of retracted stumps superior to 5 cm is recommended and often necessary [2,9]. We chose to use a semitendinosus tendon allograft of 17 cm due to the dimensions of the stump retractions of 7 and 10 cm. In doing so, it endowed the reconstructed tendon augmentation with a controlled suture tension and the correct anatomically location at the proximal insertion, thereby recreating the native insertion at the ischial tuberosity [2]. Significantly, the Y-Knot RC anchor associated with robust sutures allow loads superior to 200 N, and having this type of fixation emerged as the gold standard treatment [10]. Moreover, the longitudinal/vertical incision guided by the landmarks obtained with the aid of US and MRI gave us enough room to perform the surgery accurately, assessing the tension of the suture anchors and the sutured stumps, as well as the use of PRGF in different surgical steps. The PRGF supplied the suture anchor and sutured stumps with trophic molecules that have been reported to promote the osseointegration of the graft, a better remodeling and the secretion of extracellular matrix, while avoiding fibrosis at the suture stumps as well as around the sciatic nerve, all effects leading to enhance the repair process [4,11–13]. At this point, and following Sanchez et al. [14], one improvement to add would be to soak in and infiltrate the allograft into the PRGF supernatant. However, this novel technique is not exempt from some pitfalls, mainly stemming from the long vertical incision and the period of 8 weeks wearing the hip and knee brace, the latter being cumbersome and hard to tolerate for patients. We consider that it is not recommended to shorten this time, as the immobilization time depends on the type of tendon sutured, the type of suture and whether or not allografting is required. In our case, these are very powerful tendons that require and demand a lot of strength, and the time described should be respected to avoid the risk of dehiscence or suture failure. A long longitudinal incision is recommended in chronic proximal hamstring tears when the tear distal gap of the hamstring tendon stumps assessed by US and MRI is several centimeters [2].

### **4. Conclusions**

The PRGF-assisted reconstruction of chronic proximal hamstring tears with a semitendinosus tendon allograft provides mechanical and biological advantages to tackle the large retraction of hamstring stumps and the entrapment of the sciatic nerve within the scar.

**Author Contributions:** Conceptualization, A.R.L.; methodology, A.R.L., H.F.-S.D., M.V.M., Á.B.H., R.P., E.A. and S.P.; writing—original draft, S.P. and R.P.; writing—review and editing, A.R.L., H.F.- S.D., M.V.M., Á.B.H., R.P., E.A. and S.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** For this type of study (case report), formal ethical approval is not required.

**Informed Consent Statement:** The authors obtained the written consent of the patient for the publication of the data and images that appear in the article.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare that E.A. is the Scientific Director and S.P. and R.P. are scientists at BTI Biotechnology Institute, a biomedical company that investigates in the fields of regenerative medicine and PRGF-Endoret technology. The rest of the authors state that that they have no conflict of interest that are relevant to the content of this article.
