The results of this study showed that the level of fracture displacement was associated with postoperative pain. The degree of postoperative pain was correlated with the number of K-wires used. Intra-articular injection of 0.25% bupivacaine could effectively relieve pain in children after CRPP.
Postoperative pain in children with fractures is a serious problem. How to control postoperative pain in children safely and effectively is a recurrent goal of pediatric orthopedic surgeons and parents. Most children with supracondylar humeral fracture experienced severe pain after CRPP . It was described that the pain is most intense on the first day after surgery, and there is no clinical pain on the third day after surgery . Bupivacaine has a long half-life of up to 8 hours and studies have shown that intra-articular injection of bupivacaine could induce an analgesic effect up to 12 hours after anterior cruciate ligament surgery . Therefore, bupivacaine injection could reduce pain in children with CRPP within 24 hours after surgery.
There are several studies involving postoperative pain of supracondylar humeral fractures in children. Except for oral and intravenous analgesics , hematoma block is another commonly used pain control method for children with fractures. Bear et al.  performed local hematoma block with 1% lidocaine for children undergoing closed reduction of distal radius fracture, and Herrera et al.  performed intraoperative hematoma block with bupivacaine at the fracture site for children undergoing femoral elastic intramedullary nail. The results showed that hematoma block could relieve pain and reduce the dose of postoperative morphine, codeine and other analgesics. In a recent study, Astacio et al.  performed a 0.25% bupivacaine local hematoma block in children with humeral supracondylar fracture after CRPP. Compared with the control group, postoperative use of morphine doses and pain score were not significantly different between groups, therefore the author suggested that hematoma block is not an effective method to relieve postoperative pain of supracondylar humeral fracture . Intra-articular injection has shown good results in the treatment of pain after orthopedic surgery in adults. However, it is rarely used in the in children. Only one randomized controlled study  showed that intra-articular injection of 0.25% bupivacaine could significantly reduce postoperative pain in children with supracondylar humeral fracture after CRPP. Because the fracture line of supracondylar humeral fracture is located within the elbow joint capsule [12, 23], bupivacaine injected into the joint could be applied to the fracture site to provide an analgesic effect.
However, there are some limitations of intra-articular injections. One is the risk that intra-articular injections may cause chondrolysis. Although bupivacaine, lidocaine, ropivacaine and levobupivacaine are all toxic to cartilage, bupivacaine was shown to be the least cytotoxic . Indeed, a single injection of bupivacaine in the articular cavity does not have harmful effects on chondrocytes , therefore, the intra-articular injection of bupivacaine is considered safe. Similar to the results of previous studies, there was no case of chondrolysis in our study.
Another risk is postoperative osteofascial compartment syndrome. Osteofascial compartment syndrome is a rare but catastrophic complication after fracture in children. It often occurs in children with elbow fracture , and the incidence is less than 0.5% . Early detection is the key to avoid the occurrence of osteofascial compartment syndrome. In the control group, one child developed osteofascial compartment syndrome and ischemic contracture of the forearm, and neurotenolysis was performed 3 months after operation. The clinical symptoms of osteofascial compartment syndrome in children are not typical, especially for those children with fractures complicated with nerve injury. Attention is therefore recommended to the occurrence of fascial compartment syndrome in order to avoid limb disabilities in children . Although studies have shown that ultrasound-guided regional block and one additional shot of brachial plexus block  can relieve postoperative pain in children with supracondylar humeral fracture, these may increase the risk of osteofascial compartment syndrome. We speculate that a small dose of bupivacaine injected into the joint capsule does not increase the risk of osteofascial compartment syndrome.
Ibuprofen is the most commonly used painkiller for children in our hospital. Because of its efficacy and safety, ibuprofen is a good choice for pain relief after musculoskeletal trauma in children . Compared with other drugs such as morphine, acetaminophen and codeine, ibuprofen can effectively relieve pain after fracture in children, with fewer side effects and higher satisfaction of children and parents. Therefore, it is recommended to use ibuprofen for postoperative pain management [31, 32, 33]. One of the indicators for evaluating the efficacy of intra-articular injection was the postoperative doses of ibuprofen. The results showed that an intra-articular injection with 0.25% bupivacaine could significantly reduce the postoperative doses of ibuprofen.
The main limitation of this study is that this is a retrospective study. In addition, the surgeon was not blinded. In order to reduce possible bias, we blinded the staff responsible for statistical analysis. In the future, we will conduct a randomized controlled study to evaluate the analgesic efficacy of intra-articular bupivacaine in children with supracondylar humeral fractures with a larger sample size.
In conclusion, the intra-articular injection of 0.25% bupivacaine is a safe and effective method to significantly reduce postoperative pain following CRPP of supracondylar humeral fractures in children.