Biomechanical evaluation of straight antegrade nailing in proximal humeral fractures: the rationale of the “proximal anchoring point”

Twenty matched pairs (n = 40) of fresh-frozen human cadaveric shoulders (ten male, ten female; mean age, 82.7 years [range, 73–95]) were utilized for this study. Paired specimens were randomly distributed between two homogenous groups: group A, no PAP (five male, five female; mean age, 82.6 years [range, 73–92]; mean BMI 23.1 [range, 15.4–33.4]; mean BMD 67 mg/cm³ [range, 52–83]) and the group B, custom PAP (five male, four female; mean age, 81.9 years [range, 75–95]; mean BMI 25.2 [range, 18.8–34.7]; mean BMD 73 mg/cm³ [range, 55–88]) from highest to lowest age. The first pair was randomly assigned by flipping a coin. There were no significant differences between the groups regarding age, gender, BMI or BMD. Within no-PAP group A, subgroup A1 was assigned a common endcap (undersized, compared to the nail’s proximal tip diameter) attached to the tip of the nail, while subgroup A2 did not have an endcap. Within custom-PAP group B, subgroup B1 was assigned a common endcap (identical construct as subgroup A1), while subgroup B2 was given a custom endcap (equally sized, compared to the nail’s proximal tip diameter) (Fig. 2). One pair of humeri (group B) had to be excluded from final analysis due to incorrect potting of one humeral head, leading to instability during biomechanical testing.

To estimate the bone mineral density (BMD), computed tomography (CT) scans of all specimens were conducted. According to the method described by Krappinger et al. [7], the cancellous BMD of the proximal humeral heads, corrected for yellow fat marrow, were calculated.

Prior to preparation, specimens were thawed at room temperature for 24 hours and dissected free of all soft tissue. By visual and CT scan inspection, no prior surgeries, preexisting injuries or pathologies, other than osteoporosis were identified. According to the age, some paired specimens showed mild to severe grades of osteoarthritis.

The humeri were cut distally to a length of 175 mm and potted to a depth 135 mm distal to the humeral head’s most proximal extension in polymethyl methacrylate (PMMA; Fricke Dental International, Streamwood, IL, USA) using a custom cylindrical mold, with the long axis of the humerus concentric with the cylindrical axis of the mold. Prior to potting, screws were inserted through the shaft into the most distal 5 mm of the humeri to ensure rigid fixation in the PMMA.

According to the manufacturer’s technique guide (DePuy Synthes, West Chester, PA, USA), the dedicated entry points of the nails on the humeral heads were identified and marked in line with the shaft axis.

The SAN’s entry point was determined by projection of the physiological shaft axis at the top of the head fragment. The head was then opened using an 11.5-mm drill, and the nail (MultiLoc 9.5 × 160 mm, proximal diameter 11 mm; DePuy Synthes, West Chester, PA, USA) was implanted. MultiLoc proximal locking screws (4.5-mm MultiLoc Titanium; DePuy Synthes, West Chester, PA, USA) of the appropriate length were inserted into the humeral head, according to the manufacturer’s technique guide. The locking screws were chosen to be as long as possible but to not penetrate the humeral cortex contralaterally, and were placed in the same standardized orientation for every specimen.

Two bicortical antirotational locking screws (4.0-mm Titanium Locking; DePuy Synthes, West Chester, PA, USA) were inserted distally. All screws were placed using the manufacturer’s aiming frame. To ensure comparable positioning of the locking screws between pairs, the nail’s proximal tip was distalized 10 mm below the most superior extent of the humeral head. Although this seems to appear rather distal in some cases, it does reflect clinical reality—in tall individuals, the nail’s tip will end up to 15 mm distal to the humeral head’s most proximal cortical extension in order to place the very proximal and distal locking screw options correctly. An endcap (MultiLoc Endcap, diameter 9.5 mm [only available diameter], length 10 mm; DePuy Synthes, West Chester, PA, USA) was attached to one humerus from each pair (subgroups A1 and B1). The contralateral humerus nail’s proximal tip was left uncapped in the no-PAP group (subgroup A2), while a 10-mm-long custom endcap with a diameter of 11 mm (in accordance with the proximal nail’s diameter) was attached in the custom-PAP group (subgroup B2).

Finally, a 10-mm circumferential osteotomy was created in all specimens to imitate a two-part proximal humeral fracture with a comminuted medial calcar. The proximal cut for the osteotomy was initiated 2 mm below the most distal level of the articular cartilage. All specimens were then potted in a custom cylindrical mold to a depth of approximately 40 mm, so as to completely cover the articular cartilage. According to previously described models, the long axis of the humerus was oriented in neutral position and 20° of abduction relative to the cylindrical axis of the mold in order to apply varus dislocating force (Fig. 3) [8, 9]. Prior to potting the humeral head, two screws were inserted through the head, oriented medially to the nail to ensure rigid fixation in the PMMA.

The humeri were biomechanically evaluated using a dynamic tensile testing machine (ElectroPuls E10000; Instron Systems, Norwood, MA, USA). Before the testing of each humerus, a cylindrical metal socket with a spherical cavity for load application (via a ball attached to the testing machine actuator) was fixed within the medial side of the proximal potting (Fig. 3). Each humerus was secured within a custom fixture at the distal potting, while ensuring the proximal potting on the humeral head was level in both anterior/posterior and medial/lateral directions. In 20° of abduction, the humerus was preloaded to 15 N, which was the minimum load for subsequent cyclic loading. Using a caliper, the gap of the osteotomy was measured at the most medial location. The specimen was then cyclically loaded between 15 and 50 N at a frequency of 0.25 Hz with the maximum load increasing stepwise by 50 N every 100 cycles. After completion of each load step, the osteotomy gap was re-measured. Endpoint of testing, due to failure of the fixation, was defined as an osteotomy gap closure measurement greater than or equal to 5 mm relative to the initial gap measurement, which appeared to be equivalent to a permanent varus displacement in preliminary experiments. Implant cut out as well as any other fracture occurrence were also defined as fixation failure. Failure load was defined as the last completely sustained load magnitude during the stepwise increasing cyclic loading without failure. Following testing, all specimens were carefully freed of all potting material and examined.

To address the primary hypothesis, failure load was compared between two fixation methods of each test group separately. To reflect the fact that a failure event was defined based on measurements taken after each load step with 100 cycles, the overall load data were analyzed as an ordinal variable. Thus, medians and ranges were reported, and the Wilcoxon signed-rank test was conducted. An a priori sample size calculation indicated that ten specimens per group would be sufficient to detect an effect size of d = 1 for matched-pairs testing with 80% power. The statistical software package R (R Development Core Team, Vienna, Austria) was used for all data analysis and statistical significance was declared for p < 0.05 [10].