Functional Outcomes Following Fixation of an Ultra-distal Radius Fracture with Two Commonly used Volar Locking Plates: A Retrospective Cohort Study

Introduction: The volar locking plate has been widely used for unstable distal radius fractures to provide early recovery of wrist function. Volar plate prominence to the watershed line has been reported to be related to exor tendon irritation, and avoid implant prominence in this area was suggested. On the other hand, ultra-distal radius fracture patterns required the plate to cross the watershed line, making conict over plate positioning on ultra-distal radius fractures. This study compared functional outcomes in patients with ultra-distal radius fractures treated with two different implants. and A retrospective study was conducted, all patients who received a Synthes 2.4mm LCP or an Acumed Acu-Loc VLP between January 2015 and December 2018 were reviewed. The ultra-distal fracture pattern was the most distal horizontal fracture line within 10 mm of the lunate fossa's joint line. The primary outcomes including patient-reported pain scores, range of motion, and grip strength were assessed. Secondary outcomes included patient-based subjective satisfaction scores of the injured wrist and hand function. The Mayo Wrist Score and the requirement for a secondary procedure related to hardware complications were also recorded. Forty-two patients met our inclusion criteria. Twenty-one patients were treated with the Synthes 2.4 mm LCP, and 21 patients with the Acumed Acu-Loc VLP. The primary outcome revealed that post-operative range of motion (P = 0.016) and gripping strengths (P = 0.014) were signicantly improved in the Acu-Loc VLP group. The MAYO wrist score in the Acu-Loc VLP group was also signicantly better (P = 0.006). Despite advances in implant designs, exor tendon irritation or rupture is still a complication following distal radius's volar plating. We believe the Acumed Acu-Loc VLP design provided better functional outcomes than the Synthes 2.4 mm LCP if appropriately and carefully placed into its designed-for position. This positioning results in promising patient satisfaction when treating ultra-distal radius fractures. that position the plate over the watershed line is essential for adequate xation. Equally, an increased risk of exor tendon injury is also an issue. To the best of our knowledge, no previous reports have compared different distal radius VLP designs for ultra-distal or comminuted intra-articular fractures which increase xation over ultra-distal fragments. In this study, we investigated primary and secondary functional outcomes in patients with ultra-distal radius fractures, or comminuted intra-articular fracture patterns, treated with two different distal radius VLP designs; Synthes 2.4 mm LCP™ Distal Radius System Juxta-articular volar plates (Synthes 2.4 LCP) and the Acumed Acu-Loc Wrist Plating System Volar Distal Radius Plate (Acumed Acu-Loc VLP).


Introduction
Distal radius fractures are one of the most common skeletal injuries of the wrist, accounting for 14-18% of all fractures in adults (1,2). Most distal radius fracture occur in elderly patients, with an increasing incidence in aging populations. Many treatment methods have been described to treat these fractures, including close reduction with casting protection, percutaneous Kirschner wire reduction and xation, joint-spanning external xation, and open reduction and internal xation (ORIF) with volar and (or) dorsal plates (3).
As fracture patterns and mechanisms are widely divergent, few of the available treatment options are applicable to all patients, therefore patient-speci c treatments are required (1,3). In the last decade, surgical techniques and implant designs have advanced considerably. Since its introduction in 2000, the volar locking plate (VLP) has been widely used in patients with unstable distal radius fractures, as it provides secure immobilization, early postoperative mobility, and rapid recovery of wrist function (1,4,5).
In the past decade, several distal radius VLP mechanisms have been designed to minimize complications, and increase xation power, thereby leading to promising postoperative outcomes for distal radius fractures (4,5,8,10). However, not all VLP designs are suitable for ultra-distal radius fracture patterns in which fracture line very close to the joint line. Standard VLP designs cannot buttress the fracture fragment at standard positions, which are proximal to the watershed line (9,10). Too proximal a position of the distal radius VLP, or inadequate selection of the plate in this fracture pattern, can lead to the subsequent displacement of the distal fragment (10). Accordingly, there is con ict over plate positioning on ultra-distal radius fractures that position the plate over the watershed line is essential for adequate xation. Equally, an increased risk of exor tendon injury is also an issue.
To the best of our knowledge, no previous reports have compared different distal radius VLP designs for ultra-distal or comminuted intra-articular fractures which increase xation over ultra-distal fragments. In this study, we investigated primary and secondary functional outcomes in patients with ultra-distal radius fractures, or comminuted intra-articular fracture patterns, treated with two different distal radius VLP designs; Synthes 2.4 mm LCP™ Distal Radius System Juxta-articular volar plates (Synthes 2.4 LCP) and the Acumed Acu-Loc Wrist Plating System Volar Distal Radius Plate (Acumed Acu-Loc VLP).

Patient selection
We conducted a retrospective study by reviewing all patients who received a Synthes 2.4 mm LCP and an Acumed Acu-Loc VLP. Those patients attending between January 2015 and December 2018 were identi ed. Inclusion criteria; patients receiving distal radius fracture xation with one of the two study implants, aged > 20 years old when the fracture occurred, and amenable to a minimum follow-up of 24 months. Exclusion criteria; patients with multiple trauma, more than one skeletal fracture, previous injury over the ipsilateral or contralateral wrist, those lost to follow-up within 24 months post-operation, open fracture, and primary injury involving the wrist tendon or neurovascular structure.

Patient classi cation
Fracture con guration and patterns were classi ed radiographically by a senior orthopedic resident and rechecked by a hand surgeon. The de nition of an ultra-distal fracture pattern was assumed to be the most distal horizontal fracture line within 10 mm of the joint line of the lunate fossa, based on anteroposterior (AP) lm. Simple volar or dorsal Barton fracture patterns (AO/OTA 2R3B2/3) were excluded ( Figure 1). All pre-operation, intra-operation, and post-operation plan lms, and pre-operation computed tomography (CT), intra-operation arthroscopic images (if Arthroscopically assisted reduction and internal xation was performed) were reviewed and recorded.

Primary and secondary outcomes
The primary outcomes were; patient reported pain scores (VAS scores) and range of motion and grip strength of the injured wrist when compared to the uninjured wrist at clinical review more than two years after the operation. Patient reported range of motion and grip strength were classi ed into ve groups: 0-24%, 25-49%, 50-74%, 75-99% and 100% of uninjured wrists for each group. Secondary outcomes included patient-based subjective satisfaction scores of the injured wrist, and hand function at work, sports and social activities, and daily life (ranging from 1-10; 1 = worst/ unsatisfactory outcome and 10 = best/satisfactory outcome). The Mayo Wrist Score and the requirement for a secondary procedure related to hardware complications (i.e., hardware removal, tendon release, tendon irritation, tendon repair due to rupture, loss of reduction or mal-positioning of the plate or screws) were also recorded. All xation and arthroscope operations were performed by hand surgeons, independent of outcome measurements and statistical analyses.

Statistical analyses
Statistical analysis was performed using SPSS 21.0 software (IBM Corp., Armonk, NY, USA). Data were expressed as the mean ± standard deviation (SD).
Comparisons between groups were performed using non-parametric tests. A P < 0.05 value was considered statistically signi cant.  (Table 1). In terms of combined injuries, ulnar styloid avulsion fractures were documented in 11 patients, but only one patient was treated with a tension band wire.

Results
DRUJ stability was checked intra-operatively by the hand surgeon who performed the volar plating. Two patients required DRUJ pinning. One patient with a TFCC traumatic tear was noted during arthroscopic assist reduction and xation, and was treated with a suture anchor. Die-punch fractures were observed in four patients at pre-operative X-ray and CT.
We observed no signi cant differences in gender, age when injured, fracture patterns, distance from the most distal horizontal fracture line to the lunate fossa, and concomitant injuries, including DRUJ instability or TFCC injury.
During follow-up, six patients underwent an operation to remove the implant due to hardware-induced discomfort (irritation); four came from the Synthes 2.4 mm LCP group(19.0% of the Synthes 2.4 mm LCP group), and two came from the Acumed Acu-Loc VLP group(9.5% of the Acumed Acu-Loc VLP group). No FPL tendon or other exor/extensor tendon ruptures were observed in the groups. Similarly, no surgical site infections, loss of reduction, malunion or other complications requiring a secondary intervention were observed.
Primary outcome evaluations revealed that post-operative range of motion (P = 0.016) and gripping strengths (P = 0.014) were signi cantly improved in the Acumed Acu-Loc VLP group. In this group, 12

Discussion
In this study, we compared two different distal radius VLP designs in patients with ultra-distal radius fractures, de ned 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 de ned as "the transverse ridge that limits the concave surface of the volar radius", and was further re ned 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 de nition 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 de nitions, the watershed line has been widely used as a distal reference point for distal radius volar plating positioning to avoid exor tendon irritation, tenosynovitis and rupture (1,7,9). Soong  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 t the volar cortex of the distal radius. The low plate-and-screw pro le, 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 pro le and beveled design (17,18)]. It has been suggested this design ideally ts 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 ange design of the plate extended toward the radial styloid; even when shaved to a thinned edge, the design is believed to be related to exor 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 t to the volar cortex of the distal radius, and was less prominent on the lateral view. Similarly, the ange concern over the radial styloid was less prominent. This may reduce the risk of exor 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 xation.
As the Acumed Acu-Loc VLP is designed to be placed distal to the watershed line, exor 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 exor 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)(8)(9)(10)]. Thus, we suggest if the fracture pattern of the distal radius does not require ultra-distal xation, 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 tting it to the anatomy of the distal radius, we believe this generates better outcomes when compared with the Synthes 2.4 mm LCP.

Study limitations
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 in uenced 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 exor tendon rupture could occur anywhere between 4 and 68 months, post-operation (20).

Conclusions
Despite advances in implant designs, exor tendon irritation or rupture is still a serious complication following distal radius VLP ORIF. Avoid placing the VLP distal than the watershed line and reduced the volar prominence of the implant on lateral view are commonly suggested. But ultra-distal or comminuted intraarticular fracture patterns require more distal xation, and placing the implant more distal to the watershed line is inevitable. We believe the Acumed Acu-Loc VLP design provided better functional outcomes when compared with the Synthes 2.4 mm LCP, if appropriately and carefully placed into its designed-for position. This positioning results in promising patient satisfaction when treating ultra-distal radius fractures. Availability of data and materials: The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request Funding: There is no funding source.

Authors' contributions:
Yin-Ming Huang: Acquisition and data, analysis and interpretation of data, drafting of the manuscript, statistical analysis; Chun-Yu Chen: Conception and design, critical revision of the manuscript, statistical analysis; Kai-Cheng Lin: Critical revision of the manuscript for important intellectual content; Yih-Wen Tarng: Revision of the manuscript, supervision; Ching-Yi Liao: Acquisition and data; Wei-Ning, Chang: Revision of the manuscript, supervision. All authors have read and approved the manuscript.   Acumed Acu-Loc VLP group with designed position; C) The Acumed Acu-Loc VLP group with a more proximal position; note the plate protrusion due to plate mis-positioning.