Fractures of the arm are frequent in skeletally immature children (1, 2). During skeletal development, bones are less stable but much more elastic than in adulthood. These properties explain the higher incidence and the more rapid healing of fractures in children and adolescents. The use of Kirschner wires is widely used for the treatment of wrist and forearm fractures. Still, in the absence of reliable data regarding the passage of fixation devices through the growth plate, most surgical techniques used for treating radius and ulna fractures are based on absolute respect for the growth cartilage. To ensure a correct interpretation of the data and to specifically avoid the potential bias related to the wrist fracture healing process near the growth plate, we enrolled only patients with mid-shaft forearm fractures. These lesions are one of the most common injuries in children, being between 3% and 6% of all pediatric fractures (3, 4). Several studies have shown that this percentage has constantly increased in the last decade because of sports/spare time activities with high risk of trauma, like the widespread use of trampolines or, in younger children, inflatable games (5–7). Treating these injuries is currently controversial, and management protocols are constantly evolving. Closed reduction and cast immobilization remain the most common treatment for the majority of these fractures, especially in younger children. The operative treatment of pediatric forearm fractures should be reserved for cases in which an acceptable alignment with closed manipulation is not possible, or in the presence of neurovascular injury, open fractures, or loss of alignment in follow-up. Internal fixation as a treatment for children's forearm fractures has recently increased, and intramedullary fixation is currently the most commonly preferred technique (8, 9). Surgical treatment must respect the features of the juvenile skeleton. Although pinning across the growth plate is commonly used to treat some fractures in children, orthopedic surgeons generally try to avoid violating the physis. In this regard, the surgical technique used as the gold standard in treating diaphyseal forearm fractures in pediatric age is elastic intramedullary nails (Elastic Stable Intramedullary Nailing – ESIN) (10, 11).

Some studies showed, with a radiographic follow-up, that transphyseal pinning was not associated with early or late complications affecting the growth cartilage (12–15).

Magnetic resonance provides a non-invasive and excellent visualization of structures and the anatomical changes of the growth plate and adjacent joint in multiple planes. Jaramillo et al., along with other authors, described the utility of MRI in evaluating physeal injuries and reported that MRI could help detect minor disruptions of the growth cartilage and show early cartilaginous and vascular changes that lead to growth disturbances (16, 17).

This study aims to evaluate, through post-operative MRI, the presence of physeal disturbance in skeletally immature patients following trans-physeal wire fixation for forearm fractures, and correlate presence of damage with wire diameter and percentage of the growth plate affected by wire passage.

The mean duration of hospitalization was 2.5 days (range: 2–3 days). In 9 patients (35%), six affected by biosseus fractures and three by monostotic fractures, an open surgery reduction was necessary (all the fractures presented a translation of bone fragments >80% of the bone diameter). The mean duration of surgery was 25 min (range: 15–46 min), while the mean duration of anesthesia was 31 min (range: 24–73 min). The most extended procedures were those in which an open reduction was performed, and muscle tissue was found between the bone fragments. According to Daruwalla's criteria, clinical data underlined excellent results in most patients: 24 patients were excellent, two were good, and there were no poor results in the six-month follow-up. At the 6-month follow-up, two patients (7%) had a deficit of pronation and supination of less than 10 and 15 degrees. A residual radial and ulnar angulation value of less than 5 degrees was observed in 1 patient (6%). In 2 patients (7%), a radio-ulnar discrepancy of less than 6 mm was found. There was no radio-ulnar cross-union or heterotopic ossification visible at the x-ray follow-up. Radiographic healing was achieved in all cases at three months, with a mean consolidation time of 7 weeks (5.7–11 wk) (Table 1). One patient had a re-fracture about four months after K-wire removal. This patient was successfully treated with closed reduction and a further K-wire fixation. We had no cases of delayed union, growth disturbance, post-operative compartment syndrome, surgery complications, superficial or deep infection, or soft tissue irritation at the entry point of wire, elbow, or wrist stiffness. Peripheral vascular-nervous deficits were not observed.

The MRI study has validated the safety of the surgical technique, showing that transphyseal pinning was not associated with early or late complications affecting the distal physis of the radius. No growth plate disturbance related to the passage of the wire through the physis was detected. The study assessed the correlation between wire diameter and the percentage of lesion tolerated by the growth plate. To our knowledge, no recent studies in the literature, with a valid sample of patients, address the safety issue of transphyseal distal radius pinning with MRI evaluation.

To specifically avoid the wrist fracture healing process near the growth plate being a bias of the study, we enrolled only patients with mid-shaft forearm fractures. The surgical management of pediatric diaphyseal forearm fractures is currently controversial (21). The advantages related to the juvenile skeleton, such as remarkably rapid healing times and the minimal percentage of complications of cast immobilization (joint stiffness, loss of bone mass, functional limitations, phlebitis, Sudeck syndromes, etc.), allow for adequate compensation of traumatic deformities such as angulation and displacement and have directed almost all therapeutic choices towards the conservative treatment for many years (22, 23). The patient's age and injury site influence the fracture's healing process. The remodeling capacity depends on the proximity of the fracture to the physis: it is greater if the fracture is close to the growth plate (24). When the fracture involves the middle third of the forearm, and the patient is older than ten years, the remodeling capacity decreases, and a higher incidence of residual angulation and/or malrotation is often observed at follow-up (25, 26). Closed reduction and plaster cast immobilization are the gold standard treatments for stable and minimally displaced forearm fractures (27). This treatment is successful in most patients under 8, with no bayonet apposition or angular deformity less than 10–15 degrees (3, 28). Surgical treatment is indicated in cases of instability, open fracture, nonunion, irreducibility, severe soft tissue injury, or progressive loss of reduction in cast (27, 29). Several techniques have been proposed and practiced for diaphyseal forearm fracture treatment in children in the last two decades. Intramedullary (IM) fixation is currently the most commonly preferred method (30). The use of plates, screws, and external fixators is indicated only in limited cases (31, 32). The intramedullary osteosynthesis of pediatric forearm fractures has not been recently introduced but has become more popular in the last decade. The most widespread technique in this context is the elastic nails method (ESIN or Nancy nails) (10, 11, 33–35). This procedure respects the physis: the radius nail is implanted from the proximal metaphysis through a lateral approach (radial styloid) or a dorsal approach (Lister's tuberculum radii); it allows a limited dissection to achieve a shorter duration of anesthesia and surgery, to maintain alignment and stability of the fragments, but it requires additional anesthesia and surgery for nail removal.

The use of percutaneous pinning is an unusual method in the treatment of diaphyseal forearm fractures, whereas it is a widespread approach for treating displaced or unstable physeal or juxtaphyseal fractures in pediatric and adolescent patients. The use of temporary pinning across the physis, with smooth wires, finds consensus in the literature and is widely practiced to date (36), as is frequently done for Salter-Harris type physeal fractures (37) or Gartland II-III supracondylar humerus fractures (38, 39).

Choi et al. (1995) (40), Yung SH et al. (1998) (30), and Yung PSH et al. (2004) (41) described in their studies, respectively 157, 57, and 84 cases of pediatric forearm fractures treated by percutaneous transphyseal intramedullary Kirschner wire pinning (1.6 mm or 1.2 mm of diameter). The K-wire was introduced from the radial styloid and was pushed across the physis. Despite this, these studies have already shown, with clinical and radiographic investigations and a mean follow-up of 20 months, that transphyseal pinning was not associated with long-term outcomes affecting cartilage growth. They did not highlight nonunion, premature epiphyseal closure, or deep infection. It should be noted, however, that the potential limitation of these studies is that the follow-up was conducted only with x-rays. Şahin et al. (2017) (12) analyzed the radiographic and clinical results of pediatric diaphyseal forearm fractures treated with titanium nails or K-wires. They demonstrated no significant difference between the two procedures regarding the union time of fractures, rate of postoperative complications, range of elbow motion, and postoperative symptoms.

There is currently no consensus regarding the impact of the K-wire on physeal injury. Some studies have shown how the K-wire placed across the physis could contribute to physeal arrest (42). In the past, the relationship between transphyseal pinning and physeal growth disturbances has been a topic of investigation in several animal studies, suggesting a potential correlation between the placing of K-wire across the physis and physeal damage (13, 43). Certain factors, such as size, location, number of passes, and number and type of K-wires, may contribute to physeal injury. Other studies, however, have shown that minor central disruptions of the physis do not cause an alteration of the physeal growth (14, 44).

In the absence of reliable data regarding the passage of fixation devices through the growth plate, most surgical techniques used for treating radius and ulna fractures are based on absolute respect for the growth cartilage.

Magnetic resonance imaging is currently considered the most suitable examination for studying cartilage tissue. MRI allows an accurate, radiation-free, volumetric evaluation of the physis.

Shi DP et al., in 2009, showed that MRI accurately documents defects of physis more clearly than other exams in physeal injury (16). Jaramillo et al. described the utility of MRI in evaluating physeal injuries. They reported that MRI could be functional in minor disruptions of the growth cartilage detection and can show early cartilaginous and vascular changes that lead to growth disturbances (17).

Our MRI exam protocol included 3D Volumetric Interpolated Breath-hold Examination (VIBE), 3D Proton density (DP), and 3D T2 SPACE Short Tau Inversion Recovery (STIR), with a total exam time between 10 and 15 min. VIBE sequence is a form of volumetric imaging using fast 3D gradient-echo (GRE) sequences that produce T1 weighted images (TR 5.17 msec – TE 2.2 msec). This high intrinsic contrast resolution sequence allows differentiation between cartilage (hyperintense) and bone or bony bridges (hypointense areas). Proton density sequences provide (TR 1.100 msec, TE 41 msec) excellent bone, periosteum, and cartilage anatomic details. T2 3d STIR (TR 2.900 msec, TE 203 msec – inversion time 160 msec) is a high contrast fluid-sensitive sequence that best detects bone marrow edema. High-resolution 3D comparative MRI is essential in evaluating the radial growth plate due to its oblique orientation and regular undulation that increases with age in response to physiologic dynamic biomechanical forces.

Moreover, 3D MRI imaging can provide quantitative data about the physis and can differentiate a physiologic bridge, due to its imminent closure, from an iatrogenic bridge caused by the K-wire. T1-weighted sequences are more helpful in distinguishing the open and closed parts of the physis (45). The disadvantages of MRI are its expense, limited availability, and the necessity for sedation of younger children (for this reason, patients under six were excluded from the study).

From the descriptive statistical analysis of our study, it emerged that the significant correlations found are between the presence of bone bridge and the variables patient's age, diameter of the Kirschner wire used, and the WTPA. However, given that in all cases, the location of the bony bridge does not correspond to that of the Kirschner wire passage, the most realistic interpretation is that this alteration corresponds to the initial phase of the growth plate closure process. In a similar way, the correlation between the presence of the bone bridge and the WTPA (wrist total physeal area) may depend on the older age of patients with a wider WTPA.

Therefore, about what has just been said, considering the diameter of the K-wire used (2/2.5 mm) and that our MRI study has highlighted that the percentage of residual cartilage lesions was less than 1% in all the cases, this technique should be considered safe.

Another aspect to consider is the physiological closure of the distal growth plate of the radius. Kraus et al. (45) described in their work that the starting point of the closure of the growth plate could coincide with the area with the least blood supply of the physis. They evaluated the dynamic of the distal radius's physeal closure, and the MRI sequential assessments showed that bony bridging of the distal radius physeal plate begins centro-radial and ends with a radio-dorsal bridge (Figure 5).

Therefore, the entry point of the Kirschner wire, in the central part of the epiphysis, should reduce the risk of damage to the growth plate. The central entry point of the K-wire is necessary for its correct and easy insertion into the canal. An eccentric entry point often leads to the creation of false paths. Furthermore, the direction of the wire should be perpendicular to the growth plate and placed in the center of the medullary cavity. In this way, using a smooth fixation device, the growth plate retains the ability to grow physiologically, minimizing the possibility of growth disturbances. On the contrary, the growth process might be altered when the K-wire is fixed obliquely in the cortical bone. Moreover, the oblique orientation would involve a larger portion of the physis.

Lastly, another aspect worth considering is the shape of the radio-carpal joint, which consists of two independent articular surfaces, the scaphoid facet (laterally) and the lunate facet (medially). Because of that, the K-wire central entry point allows any possible damage to only hit a small peripheral portion of one or the other. Moreover, premature epiphyseal closure is rare despite the frequency of physeal fracture of the distal radius, unlike distal femoral epiphyseal separations, which are less common but have a high rate of complication (46).

According to Dorman et al., the K-wire fixation did not increase the risk of growth arrest (15).

In their 2008 study, Smith et al. (47) analyzed five cases of pediatric unstable upper extremity fractures were treated with transphyseal pinning, which included two Gartland type III supracondylar humerus fractures, one Gartland type II supracondylar humerus fractures fracture, one both bone forearm fracture, one distal radius physeal fractures. They evaluated the physis using MRI at three and six months after the removal of the K-wire, identifying only one case of physeal bridging that was believed to be related to the trauma (a Salter-Harris type II fracture of the distal radius). Therefore, this study, despite being limited by the small sample of patients, has shown that temporary pinning across the physis did not necessarily cause physeal damage.

Our study has limitations, including the retrospective nature and a relatively small number of patients. However, the study results confirmed that transphyseal K-wire could be safely used, and it is a valid and minimally invasive surgical technique. In some cases, transphyseal intramedullary fixation could be an alternative surgical treatment for forearm shaft diaphyseal and distal radius fractures. It is easy to perform, has several advantages, and has a low complication rate. Furthermore, the percutaneous technique does not require the skin incision for surgical exposure and additional surgery and/or anesthesia for wire removal.

The MRI study has validated the safety of this surgical technique and confirmed that the distal radius growth plate better tolerates injury compared with other physis. It has demonstrated the absence of long-term damage to the cartilage. It confirms, as other studies, that the growth plate could be violated if some technical and biological criteria are respected:

− To use a non-threaded K-wire;