In this study, we compared two different distal radius VLP designs in patients with ultra-distal radius fractures, defined as the most distal horizontal fracture line lying within 10 mm of the lunate fossa. Both VLP are widely used volar plating systems designed for fracture very close to the joint, while the fracture pattern required more distal placement of the plate, namely, to cross the watershed line.
The watershed line concept was proposed by Orbay in 2005, and was defined as “the transverse ridge that limits the concave surface of the volar radius”, and was further refined by Nelson and Orbay, as ”the theoretical line marking the most volar aspect of the volar margin of the radius” (11, 12) Thus, there is no generally accepted definition of the watershed line; however, other interpretations include: “The distal radial physeal scar”, “The distal border of the pronator quadratus muscle” and “The origin of the volar carpal ligaments” (13).
In spite of these disparate definitions, the watershed line has been widely used as a distal reference point for distal radius volar plating positioning to avoid flexor tendon irritation, tenosynovitis and rupture (1, 7, 9). Soong et al. defined the watershed line as” the most prominent part of the volar surface of the distal part of the radius, where the flexor tendon lies closest to the plate and bone” . In 2011, these authors reported the clinical relevance of the watershed line with flexor tendon complications, by introducing “Soong Grading” of the plate position of distal radius volar plating. By placing the distal radius VLP close to or across the watershed line, this resulted in Soong Grade I or II positioning on post-operation lateral views, and was related to a higher incidence of flexor tendon complications, such as tendon irritation, fraying and rupture (4, 6, 9) Drobetz and Kustscha-Lissberg et al. reported FPL rupture in 12% of patients after volar plating with an early designed implant (14). Mehrzad et al. also reported that seven of their 60 patients (12%) with implant- related complications, required secondary surgical interventions (1). Even with modern low-profile anatomical implant designs, flexor tendon injury incidences of up to 2% have been recorded (4, 6, 15, 16).
To avoid or limit complications, Soong et al. recommended that surgeons avoid implant prominence at the watershed line, however, plate positioning during surgery must account for patient anatomy, fracture patterns and implant design, as well as fitting into the contours of the volar distal radius. However, attempts to maintain the VLP proximal to the watershed line, and not protrude over the most prominent part of the volar surface on the lateral view (Soong grade 0) are not always achievable (8-10).
When treating patients with ultra-distal fracture patterns, placing the VLP distal to the watershed line is inevitable, and makes plate positioning a challenge during operations. Several implants designs have been designed precisely for these scenarios, i.e., the distal edge of the plate is polished, beveled and contoured, or a notch is placed over the trajectory of the FPL to avoid irritation and reduce pressure beneath the FPL tendon (4, 8, 10).
Both study implants were designed for far distal or intra-articular fractures of the distal radius, and to sit distal to the watershed line, but both have with different solution approaches to avoid tendon irritation (17, 18). The Synthes 2.4 mm LCP™ Distal Radius System Juxta-articular volar plate is pre-contoured to fit the volar cortex of the distal radius. The low plate-and-screw profile, round plate edges and undercut of the plate-head facilitates intraoperative contouring of the plate, based on individual patient anatomy and fracture patterns (Figure 2) (17)
In contrast, the Acumed Acu-Loc Wrist Plating System Volar Distal Radius Plate has a more rigid and complex pre-contoured design of the distal edge, and is based on the modern module of general population distal radius anatomy. It also comes with a round plate edge, low profile and beveled design (17, 18)]. It has been suggested this design ideally fits the watershed line (cadaver study), whereas the manufacturer states it is “designed to be placed more distal then many other volar plates” (Figure 2) (18, 19).
In previous studies, the incidence of FPL iatrogenic injury after Acumed Acu-Loc VLP use has been widely reported (4, 6, 7, 9). A common concern is the flange design of the plate extended toward the radial styloid; even when shaved to a thinned edge, the design is believed to be related to flexor pollicis longus tendon complications (6).
In our study, we observed no FPL tear complications. The Acumed Acu-Loc VLP patient group reported better gripping power, improved range of motion and Mayo wrist scores. These data indicated that with good Acumed Acu-Loc VLP positioning, patient range of motion was not limited, and plate related tendon irritation disappeared. Both designs made it easier for patient to recover gripping powers and improve wrist functions. We also demonstrated plate position and implant prominence in a distal radius bone model (Figure 3). When compared with the Synthes 2.4 mm LCP, the Acumed Acu-Loc VLP was a better fit to the volar cortex of the distal radius, and was less prominent on the lateral view. Similarly, the flange concern over the radial styloid was less prominent. This may reduce the risk of flexor tendon complications, resulting in better post-operative functional outcomes.
Furthermore, our bone model (Figure 3) also indicated if the Acumed Acu-Loc VLP was not placed in its designed-for position, even with a more proximal position not crossing or sitting on the watershed line, this would result in increased prominence. Thus, if the Acumed Acu-Loc VLP is to be used, the surgeon should put the locking plate on the watershed line, even if the fracture pattern does not require ultra-distal fixation.
As the Acumed Acu-Loc VLP is designed to be placed distal to the watershed line, flexor tendon complications are potential risks, post-operation. By carefully placing the Acumed Acu-Loc VLP in its designed-for position, these risks can be reduced, resulting in improved functional outcomes.
However, recent studies have reported correlations between the risk of flexor tendon irritation and implant position that crosses the watershed line and implant prominence to the volar rim of distal radius on lateral view (4, 7-10)]. Thus, we suggest if the fracture pattern of the distal radius does not require ultra-distal fixation, the implant that design to stay proximal to the watershed line should be chosen to maximally reduce complications. However, if placing the plate distal then watershed line is inevitable, such as the ultra-distal fracture pattern in this study. In choosing the Acumed Acu-Loc VLP and carefully fitting it to the anatomy of the distal radius, we believe this generates better outcomes when compared with the Synthes 2.4 mm LCP.
Our study had several limitations. It was retrospective in nature, therefore patients were not randomized, and hand-surgeon implant prevalence and operational techniques were not be standardized. The primary outcomes were based on patient report outcomes (PROs), and potentially limit objectivity, as patient expectations and compliance may have influenced the outcomes, besides implant selection. Patient numbers were relatively low, however we must also account for the relatively low incidence of ultra-distal radius fracture patterns, therefore we believe our cohort size was appropriate and acceptable. The minimal follow-up period was 24 months post-operation, and was considered adequate in capturing bone healing indices, however some delayed complications may not have been fully ascertained. Asadollahi et al. reported that delayed flexor tendon rupture could occur anywhere between 4 and 68 months, post-operation (20).