Objective
Helical plating is a known concept in humeral fracture treatment. Attention should be paid to the axillary nerve when inserting a plate underneath the deltoid muscle. The purpose of this cadaveric study was to estimate axillary nerve stretching when introducing the plate. Methods: On 42 fresh frozen human humeri, an 8-, 10- and 12-hole Philos plate in a straight and a helical shape were compared measuring the maximum plate-bone-distance. Results: For all three plate lengths, the helical plates had a significantly lower plate-bone-distance. Conclusion: Indirectly, this suggests a lower axillary nerve elongation and hence less chance of nerve damage.
Keywords: Humeral fracture, Helical plating, Fracture fixation, Axillary nerve, Nerve injury
1. Introduction
Plate-screw-osteosynthesis is frequently used to treat proximal humeral or humeral shaft fractures by orthopaedic trauma surgeons worldwide. Yet it remains a demanding surgical technique concerning reduction and fixation of the fracture. Some authors reported complication rates up to 40% with plating of proximal humeral fractures. Complications vary from nerve damage over implant failure up to avascular necrosis of the humeral head.1, 2, 3, 4, 5, 6 Recent development of surgical techniques and implant refinement led to the use of the angular stability concept which provides the opportunity to fixate osteoporotic bone and the possibility to leave periosteal bone vascularisation intact.7 Also, surgical techniques ameliorated. Since the use of the minimally invasive plate osteosynthesis (MIPO) technique in humeral fractures, authors reported lower complication rates using this technique compared to the conventional open plating regarding soft tissue damage and potential nerve injury.8,9 Hereby, a minimal invasive duplex approach to the proximal and distal humerus was described by splitting the deltoid muscle laterally and the mid third and distal humerus anteriorly. Therefore, the use of pre-contoured anatomically twisted helical plate osteosynthesis is a known concept in treatment of humeral fractures.10, 11, 12 The lateral aspect of the proximal humerus is ideal for screw fixation to the neck and head of the humerus. The anterior or anteromedial surface is the perfect position for plate application at the mid third and distal part of the bone, taking the anatomy of the humerus into account.11 A helically shaped implant offers the possibility to place the device on these different aspects of the bone. However, with the MIPO technique and insertion of the plate underneath the deltoid muscle against the surface of the proximal humerus, a higher rate of axillary nerve injury was described compared to the conventional open technique.13 In our hypothesis, using a helical plate could stretch the nerve less than a straight implant and therefore minimize the risk of axillary nerve injury. The purpose of this cadaveric study was to estimate axillary nerve stretching when inserting helical and straight implant devices in a subdeltoid plane.
2. Materials and methods
2.1. Sample and measurement pattern
The study sample included 42 fresh frozen adult human cadaveric humeri which had been embalmed by use of Thiel's method at the Institute of Anatomy of Medical University Graz.14 Since local soft tissues had been removed gender identification was not possible. Bones showing obvious signs of healed fractures or interventions were not included in this study.
First, the humeral length (HL) was measured, defined as the interval between the distal tip of the lateral epicondyle and the apex of the greater tubercle. For testing of the plate positions, an 8-, 10- and 12-hole Proximal Humerus Interlocking System (Philos) plate (Synthes® GmbH, Oberdorf, Switzerland) was utilized on each of the bones. The respective plate length was used as a straight and a helical version. The helical version was bent at its middle third in a ventral direction at an angle of about 70–90° over a distance of two plate holes. This same technique was applied to all plates in order to shape them helically. All three plates were applied in the helical and the straight version to the humeri from cranial to caudal underneath the deltoid muscle against the surface of the proximal humeral bone. No screw fixation to the bone was applied. The highest distance between the bone and the plate (plate-bone-distance) was measured (Fig. 1). Finally, it was noted which plate hole (counted starting from the distal part of the plate) matched with the highest distance to the bone (Fig. 1). Fig. 2 shows how the plate was bent and wrapped around the humerus to cover and fit the bony surface as good as possible.
Fig. 1.
This figure shows a left proximal humerus. The plate-bone-distance is indicated with a black line between the bone and the plate.
Fig. 2.
This figure illustrates, in an anteroposterior and lateral view, how the plate is bent and wrapped around the bone.
2.2. Statistical analysis
The data were analysed using the statistical software IBM SPSS®Statistics (Version 23). A descriptive statistics analysis was performed. As a normal distribution of the data could be proved, the T-test was applied. A p-value of <0.05 was regarded significant.
3. Results
The mean humeral length (HL) of the 42 specimens was 29.1 cm (SD: 1.79; range: 26–35).
3.1. Plate-bone-distance
The mean plate-bone-distance for the 8-, 10- and 12-hole straight plate was 11.1 mm (SD: 1.6; range: 5.9–16.1) while it was significantly lower (p < 0.005) for the helical plate with 8.7 mm (SD: 1.7; range: 5.1–14.5) (Table 1).
Table 1.
Descriptive statistics for plate-bone-distance (distances are measured in millimetres).
| Plate-Bone-Distance | Straight Plate | Mean | 11.0860 | |
| 95% Confidence Interval for Mean | Lower Bound | 10.8081 | ||
| Upper Bound | 11.3638 | |||
| Median | 10.9250 | |||
| Variance | 2.484 | |||
| Standard Deviation | 1.57597 | |||
| Minimum | 5.91 | |||
| Maximum | 16.09 | |||
| Range | 10.18 | |||
| Helical Plate | Mean | 8.7456 | ||
| 95% Confidence Interval for Mean | Lower Bound | 8.4463 | ||
| Upper Bound | 9.0449 | |||
| Median | 8.7050 | |||
| Variance | 2.882 | |||
| Standard Deviation | 1.69758 | |||
| Minimum | 5.05 | |||
| Maximum | 14.49 | |||
| Range | 9.44 | |||
All the three different lengths of the helical plate showed a lower mean plate-bone-distance than the three plate lengths in the straight version. Considering the helical design, the 8-hole plate had the lowest mean plate-bone-distance while the 10-hole plate had the highest (8.5 mm, SD: 1.9 vs. 9 mm, SD: 1.9) (Table 2). Regarding the straight plates, the 8-hole plate showed again the lowest mean plate-bone-distance among the various lengths and the 10-hole plate the highest (10.5 mm, SD: 1.4 vs. 11.5 mm, SD: 1.5) (Table 3).
Table 2.
Characteristics of helical plates (distances are measured in millimetres).
| 8-hole Philos plate |
10-hole Philos plate |
12-hole Philos plate |
||||
|---|---|---|---|---|---|---|
| Plate-bone-distance | Hole-highest distance | Plate-bone-distance | Hole-highest distance | Plate-bone-distance | Hole-highest distance | |
| mean | 8.5 | Nb. 5: 100% | 9.0 |
Nb. 5: 93% Nb. 4: 7% |
8.7 |
Nb. 5: 64% Nb.4: 36% |
| SD | 1.9 | 1.9 | 1.1 | |||
| median | 8.6 | 9.1 | 8.6 | |||
| min | 5.1 | 5.6 | 6.2 | |||
| max | 12.3 | 14.5 | 10.8 | |||
Nb. = number, SD = Standard Deviation.
Table 3.
Characteristics of straight plates (distances are measured in millimetres).
| 8-hole Philos plate |
10-hole Philos plate |
12-hole Philos plate |
||||
|---|---|---|---|---|---|---|
| Plate-bone-distance | Hole-highest distance | Plate-bone-distance | Hole-highest distance | Plate-bone-distance | Hole-highest distance | |
| mean | 10.5 |
Nb. 5: 81% Nb. 4: 19% |
11.5 |
Nb. 5: 64.3% Nb. 4: 33.2% Nb. 6: 2.4% |
11.2 |
Nb. 5: 76% Nb.4: 24% |
| SD | 1.4 | 1.5 | 1.6 | |||
| median | 10.5 | 11.3 | 11.1 | |||
| min | 7.8 | 8.4 | 5.9 | |||
| max | 14.2 | 16.1 | 14.4 | |||
Nb. = number, SD = Standard Deviation.
3.2. Hole-highest distance
No matter which plate design or plate length was used, the majority of the plates showed the highest plate-bone-distance over hole number 5, counted starting from the distal part of the plate. Considering the helical plates, the 8-hole plate had in every specimen (100%) the highest distance to the plate over hole number 5 while it was 93% with the 10-hole plate (vs. 7% over hole number 4) and 64% with the 12-hole plate (vs. 36% over hole number 4).
For the straight plates, the 8-hole plate had 81% of the cadavers with the highest distance to the plate over hole number 5 and 19% over hole number 4. The 12-hole straight plate had 76% of the cadavers with the highest distance to the plate over hole number 5 and 24% over hole number 4. The 10-hole straight plated also showed the highest distance to the plate in 64.3% over hole number 5 vs. 33.2% over hole number 4 and 2.4% over hole number 6.
4. Discussion
The aim of this cadaveric study was to evaluate axillary nerve elongation when inserting helical and straight plates for humeral fractures. Helical plates provide a better fit to the bony surface of the humerus taking its anatomy into account.12 Helical plates are designed to provide stable internal fixation for spiral fractures in long bones caused by torsional loading. One of the main reasons for success of these implants is their feasibility to wrap around the bone and hold bone fragments together. Thereby the stiffness and stability of the fractured bone is enhanced.12
Research and analyses of Krishna et al.12 showed the biomechanical basis of the advantages of helical plate fixation over straight plate fixation for oblique and spiral fractures. First of all, at the fracture interface, the gap closure is better in helical plate fixation compared to straight plate fixation for all loading conditions. Secondly, stress shielding is reduced due to the helical shape of the plate shifting the neutral axis into the bone. Third, helical plates are most useful in spiral fractures since the implant absorbs the tensile stresses caused by torsion. Furthermore, the screws in helical plates incline in different directions providing more screw holding power than in straight plates. However, it is very important to notice that helical pre-contoured implants are not commercially available (yet). At this very moment, plates have to be bent manually in the operating theatre with plate benders. This brings along a higher risk for implant device damage and can eventually result in fixation and/or implant failure.12,15
From a clinical point of view, Fernandez was the first surgeon to use helical plates for internal fracture fixation.10 Helical plating of the humerus involves anatomically pre-contouring a plate in a way that the proximal part lies on the greater tuberosity laterally, and the distal part bent 90° to lie on the anterior or anteromedial surface of the distal humerus. Anatomically, anterior plating of proximal humeral fractures is not ideal since the plate position is hindered by the bicipital groove and proximal biceps tendon. As mentioned above, in the present study the helical plates were bent at their middle third in a ventral direction in an angle of about 70–90° over a distance of two plate holes. Fig. 2 shows how the plate is bent and wrapped around the humerus to cover the bony surface.
The axillary nerve is located circa 3.5 cm distally from the tip of the greater tuberosity.16 Care should be taken to the nerve since this structure is at risk when performing a deltoid split approach. On the other hand, cadaveric studies have shown that using the MIPO technique axillary nerve complications can be avoided.17 In the present study, the authors assumed that the nerve stretches when the plate is inserted underneath the deltoid muscle alongside the proximal humeral bone which can cause nerve damage. The results of the current study suggest a highly significant difference in axillary nerve stretching between the use of a helical versus a straight Philos plate. If a straight plate was inserted under the deltoid muscle the mean distance between the greater tuberosity and the plate was 11.1 mm compared to a mean of 8.9 mm using helical plates (Table 1). This finding was significant (p < 0.005) and concerned all three plate lengths. Considering the helical design, the 8-hole plate had the lowest mean plate-bone-distance while the 10-hole plate had the highest (Table 2).
This cadaveric study has its limitations. A major limitation was that the actual stretch on (or elongation of) the axillary nerve could not be measured directly. Therefore, the plate-bone-distance was defined suggesting a higher nerve stretching when a higher plate-bone-distance was measured. Another limitation was the bending of the Philos plates. This maneuver was always performed according to the same technique, but it remains a procedure that could not be standardized. On the other hand, in clinical setting, the plate bending is also not standardized or reproducible and depends on the anatomy of the patient, experience and insight of the surgeon.
5. Conclusions
Engineering studies recommend helical plating for spiral and oblique fractures of the humerus for biomechanical reasons. From an anatomical point of view, the use of a twisted plate lying proximally lateral against the greater tuberosity of the humerus and distally covering the ventral side of the bone makes more sense. The present cadaveric study implies an extra advantage of helical plating for humeral fractures regarding the axillary nerve. Pre-contoured helical Philos plates showed a significantly lower plate-bone-distance compared to straight Philos plates for all plate lengths tested. Indirectly, this suggests a lower axillary nerve elongation and hence less chance of nerve damage.
Informed consent
Body donors gave their written informed consent during their lifetime.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of competing interest
None.
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