Selection of intramedullary nail
Humerus’s marrow cavity was irregular in shape. Marrow cavity starts at about 10/30-11/30 of humerus, and ends at 26/30-28/30 . At sectional view, marrow cavity is round-shaped with outer layer of compact bone and inner layer of cancellous bone. Inside cancellous bone lies the pure marrow cavity. Effective marrow cavity includes both pure marrow cavity and cancellous bone. After fixation, nail body is in pure marrow cavity; and screw part is in effective marrow cavity. Thickness of cancellous bone is 1.5 ~ 2mm, enough to include 1 ~ 1.5mm screwing part of distal intramedullary nail. Therefore, when rotating the nail body, screw thread is effective in pressing the fracture site. Additionally, marrow cavity is very slim or disappears at 25/30 of humerus. Therefore, when the thread enters 25/30 of humerus, it is most effective for pressing, and can have a very good control of bone to fix the fracture. So it is very important to select the right length of intramedullary nail beforehand. It would better to compare ample choices of nails under the real-time X-ray with the actual target. Do not compare with the X-ray film because X-ray film usually has distortion, leading to discomfort length, loosened fixation or inadequate pressure, etc., proximal humerus has no bone marrow cavity. Humeral body has 135? angle with the humeral head . Humeral anatomical neck has 45? angle with humerus body. In order to make firm locking, we designed a 45? angle ramp lock structure at the end of nail, making the nail go through the anatomical neck and, therefore, can fix the intramedullary firmly. After the nail is nailed in, the hole for locking nail should be at the same plane of anatomical neck. Therefore, the length of nail also determines its firmness. Intramedullary nail uses distal cancellous bone screw to press in order not to harm the inner cancellous bone. This means rotated forwarding without expanding the cavity. It would maintain the integrity of bone membrane, and is conducive to fracture healing. This additionally requires appropriate length and thickness of intramedullary nail. Thickness should in general be 2-3mm thinner than effective marrow cavity. At least, nail body should not be over pure marrow cavity; and screw thread should not be over effective cavity.
Principles of fixation: SPCLIN has four principles
(1) Intramedullary running-through effectively prevents lateral and angular displacement of fracture; (2) cancellous bone screw of enlarged nail tail and nail tip forms axial pressure in rotation, leading to tight intercalation of fracture sites. This prevents rotation of fracture sites; (3) proximal locking screw and distal cancellous bone screw can prevent stretching and separation of fractures sites, which can also prevent rotation of fracture sites ; (4) humeral neck of proximal humerus is firm compact bone, which can be used by locking nail to nail securely intramedullary nail. Distal 1/4 bone marrow cavity is irregular and surrounded by a lot of trabecular staggered bone structure. This is inner cancellous bone, and can be used for fixation of distal screw thread. Therefore, SPCLIN can fix humeral fractures very well.
Intensity of fixation
The primary purpose of fixation is to resist against displacement and over-activity of fracture sites and to maintain matching status of fracture fragments to provide the necessary conditions for fracture healing. Traditional Chinese medicine treated middle humerus stable fracture with external splinting by a small plate. It is a widely-acknowledged effective solution to the fixation problem in fracture healing. Compared to wooden splinting, steel intramedullary nail has much greater mechanical strength. Since the application of strong internal fixation, internal fixation is always thought to be better when it is stronger. However, clinical practice has proved that excessive strength leads to stress shielding, bone resorption, even non-healing, or fracture etc. Under normal circumstances, maximum reaction force of the elbow is 10 to 22 times of holding force. When an adult is wearing clothes or having meals, reaction force of elbow is 300N; and 340N for standing up from the chair. But the reaction force is much smaller in the middle of the humerus. From our mechanical test data and analysis, SPCLIN can meet the mechanical requirements of humerus internal fixation.
According to recent literature, pressed steel splinting has 75% excellent healing rate for humerus fractures. Ender nailing has excellent healing rate of 82.6%. Intramedullary nail with locking has 96.4% excellent healing rate . In our 1006 SPCLIN cases, 755 were healed in eight weeks; six months follow-up showed no non-healing, suggesting a very good therapeutic effect. This might be attributed to the following factors: (1) we don’t expand marrow cavity when using the nail; nail body is 1mm thinner than pure marrow cavity. Thread part is 1-2mm thinner than effective marrow cavity. It does not harm the inner periosteum, maintaining very good conditions for the fracture healing. Gu et al., hold the view that “intramedullary nails currently used are cylindrical; nails come very close bone pitch, leading to hindered blood circulation and elevated intramedullary pressure . This affects the heart-ward blood coming from the periosteum, and, as a result, is detrimental to the healing of fracture.” (2) Through rotating the front screw, the enlarged nail tail can press the fracture sites, which is beneficial to fracture healing; (3) after fixed, SPCLIN doesn’t require other supplemental external fixations. So patients can have early shoulder and elbow functional training, which might improve the blood circulation and contribute to fracture healing.
Advantages of SPCLIN
SPCLIN is simple in structure, and has only intramedullary and locking nails. It is easy to learn, understand and use. Intramedullary nails currently used such as G-K nail, K-S nail, fish mouth interlocking nail, grooved interlocking nail and bifurcated interlocking nail all require open reduction . SPCLIN has sharp tip that can be used for spin entrance. When locking, it doesn’t require extra incision, and, therefore, has no need for complicated procedures associated with other nails. In general, a G-K-type intramedullary nailing takes 1.5 to 2 hours to install. However, SPCLIN takes only 20 minutes, greatly saving doctors’ labor and patients’ pain. Price of SPCLIN is as low as 200-300 Chinese Yen compared with other locking intramedullary nails that are usually priced above 3,000 Chinese Yen (domestic production). In addition, both nailing and removal are percutaneous, largely saving the cost of patients.
Efficacy of SPCLIN has a lot to do with its length. Be sure to choose one with appropriate length and thickness. The nail tail should not be too much over greater tuberosity, so as not to affect shoulder’s functional activities. Entrance point should be accurate at greater tuberosity that is close to inter-tuberosity groove and close to center (Figure 6). Do not enter at muscle’s ending point, avoiding damage to the muscle. For comminuted fracture of long segment, do not add too much pressure, so as to avoid increased separation, overlapping or displacement of bone fragments. When removing intramedullary nails, be sure to identify the exact location; do not wander around, so as not to damage the shoulder cuff and impact shoulder’s function.
Figure 6: Schematic diagram of entrance point of nailing.