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Establishing safe zones to avoid nerve injury in the posterior minimally invasive plate osteosynthesis for humerus fractures: a magnetic resonance imaging study

  • Julio J. Contreras
    Correspondence
    Corresponding author: Julio J. Contreras Fernández, MD, Pontifical Catholic University of Chile, Department of Orthopedics and Trauma, Universidad de Chile, Instituto TraumatológicoPocuro #2170, D63, Santiago (PC 7510664), Chile.
    Affiliations
    Shoulder and Elbow Unit, Pontifical Catholic University of Chile, Santiago, Chile

    Department of Orthopedics and Trauma, Pontifical Catholic University of Chile, Santiago, Chile

    Shoulder and Elbow Unit, Instituto Traumatológico, Santiago, Chile

    Department of Orthopedics and Trauma, Universidad de Chile, Santiago, Chile
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  • Arturo Meissner
    Affiliations
    Shoulder and Elbow Unit, Pontifical Catholic University of Chile, Santiago, Chile

    Department of Orthopedics and Trauma, Pontifical Catholic University of Chile, Santiago, Chile
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  • Alfonso Valenzuela
    Affiliations
    Shoulder and Elbow Unit, Pontifical Catholic University of Chile, Santiago, Chile

    Department of Orthopedics and Trauma, Pontifical Catholic University of Chile, Santiago, Chile
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  • Rodrigo Liendo
    Affiliations
    Shoulder and Elbow Unit, Pontifical Catholic University of Chile, Santiago, Chile

    Department of Orthopedics and Trauma, Pontifical Catholic University of Chile, Santiago, Chile
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  • Rodrigo de Marinis
    Affiliations
    Shoulder and Elbow Unit, Pontifical Catholic University of Chile, Santiago, Chile

    Department of Orthopedics and Trauma, Pontifical Catholic University of Chile, Santiago, Chile
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  • Claudio Calvo
    Affiliations
    Shoulder and Elbow Unit, Pontifical Catholic University of Chile, Santiago, Chile

    Department of Orthopedics and Trauma, Pontifical Catholic University of Chile, Santiago, Chile
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  • Francisco Soza
    Affiliations
    Shoulder and Elbow Unit, Pontifical Catholic University of Chile, Santiago, Chile

    Department of Orthopedics and Trauma, Pontifical Catholic University of Chile, Santiago, Chile
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Open AccessPublished:August 14, 2022DOI:https://doi.org/10.1016/j.jseint.2022.08.003

      Background

      Safety zones to avoid nerve injury at proximal incision of posterior minimally invasive plate osteosynthesis for humerus fracture have been scarcely studied. The purpose of this study was to describe the location of axillary and radial nerves (RN) in magnetic resonance imaging to establish safety zones.

      Methods

      Fifty-two magnetic resonance imaging studies of the entire humerus were reviewed. The mean age was 50.6 ± 12.1 years, with 37 female patients. The distance of the axillary nerve (AN; distal portion, humeral midpoint) and RN (medial border, midpoint, and lateral border of the humerus) was measured in relation to the posterolateral acromion angle, acromioclavicular axis, and transepicondylar axis. Univariate analysis (Student’s t test) and a multivariate analysis (linear regression) were performed. P values < .05 were considered significant.

      Results

      The AN location at the humerus was 54.9 ± 6.4 mm (20.1% humeral length [HL]) in relation to posterolateral acromion angle and 63.2 ± 6.1 mm (23.2% HL) in relation to acromioclavicular axis. The RN location was 100.2 ± 17.1 mm (36.6% HL) at the humerus medial border, 118.0 ± 21.5 mm (43.1% HL) at the humerus midpoint, and 146.0 ± 24.4 mm (53.6% HL) at the humerus lateral border. In relation to transepicondylar axis, it was 175.4 ± 15.6 mm (64.3% HL), 156.0 ± 19.0 mm (57.2% HL), and 127.4 ± 21.2 mm (46.7% HL), respectively. Nerves location was related to HL, independent of gender.

      Conclusion

      The main finding of our study is that the location of the AN and RN in relation to the humerus is related to the HL and can be used to predictably define the safe zones to avoid nerve injury in the proximal incision of posterior minimally invasive plate osteosynthesis for humerus fractures.

      Level of evidence

      Keywords

      Minimally invasive plate osteosynthesis (MIPO) has advantages over conventional techniques.
      • Toogood P.
      • Huang A.
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      Minimally invasive plate osteosynthesis versus conventional open insertion techniques for osteosynthesis.
      MIPO emphasizes minimal soft tissue dissection, indirect reduction techniques, and bridge plate fixation, improving soft tissue management and preservation of blood supply.
      • Toogood P.
      • Huang A.
      • Siebuhr K.
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      Minimally invasive plate osteosynthesis versus conventional open insertion techniques for osteosynthesis.
      MIPO for humeral shaft fractures yields functional outcomes similar to those of open reduction, with significantly less blood loss, shorter operative duration, and a lower incidence of nonunion, radial nerve (RN) palsy, and infection.
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      Is minimally invasive plating osteosynthesis better than conventional open plating for humeral shaft fractures? A systematic review and meta-analysis of comparative studies.
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      • Hohmann E.
      • Glatt V.
      Minimally invasive plate osteosynthesis of humeral shaft fractures: current state of the art.
      In distal third humeral shaft fractures, single column osteosynthesis, using the triceps sparing posterolateral approach, has reported excellent functional results and low complication rates.
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      Outcome of extra-articular distal humerus fractures fixed by single column extra-articular distal humerus locking compression plate using triceps sparing postero-lateral approach.
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      • Pandit R.
      Retrospective analysis of extra-articular distal humerus shaft fractures treated with the use of pre-contoured lateral column metaphyseal LCP by triceps-sparing posterolateral approach.
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      • Lee H.M.
      • Chun Y.M.
      Anatomic fit of precontoured extra-articular distal humeral locking plates: a cadaveric study.
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      • Caba-Doussoux P.
      Surgical treatment of extra-articular distal-third diaphyseal fractures of the humerus using a modified posterior approach and an extra-articular plate.
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      • Mehta S.
      Plate selection for fixation of extra-articular distal humerus fractures: a biomechanical comparison of three different implants.
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      • Agrawal P.
      • Das S.
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      • Kumar A.
      Functional outcome of extra-articular distal humerus fracture fixation using a single locking plate: a retrospective study.
      A cadaveric study demonstrated the safety and feasibility of using an extra-articular distal humerus plate with the posterior MIPO technique.
      • Gallucci G.L.
      • Boretto J.G.
      • Vujovich A.
      • Alfie V.A.
      • Donndorff A.
      • De Carli P.
      Posterior minimally invasive plate osteosynthesis for humeral shaft fractures.
      The risk of RN injury can be minimized through careful dissection at proximal incision, considering that the nerve crosses the medial border of the humerus at 31.7%-45.6% of the humeral length (HL). Gallucci et al reported clinical results for posterior MIPO with segmental isolation of the RN.
      • Gallucci G.L.
      • Boretto J.G.
      • Alfie V.A.
      • Donndorff A.
      • De Carli P.
      Posterior minimally invasive plate osteosynthesis (MIPO) of distal third humeral shaft fractures with segmental isolation of the radial nerve.
      ,
      • Gallucci G.L.
      • Boretto J.G.
      • Vujovich A.
      • Alfie V.A.
      • Donndorff A.
      • De Carli P.
      Posterior minimally invasive plate osteosynthesis for humeral shaft fractures.
      The proximal incision was made in the posterior aspect of the arm, 10 cm distal to the posterolateral angle of the acromion (PLAA), and only one patient (4.8%) developed transient RN palsy.
      • Gallucci G.L.
      • Boretto J.G.
      • Alfie V.A.
      • Donndorff A.
      • De Carli P.
      Posterior minimally invasive plate osteosynthesis (MIPO) of distal third humeral shaft fractures with segmental isolation of the radial nerve.
      However, the safety zones in the proximal incision relative to the position of the axillary and RNs have been rarely studied. The purpose of this study was to describe the location of the axillary and RNs in magnetic resonance imaging (MRI) of the humerus and to establish safety zones for the proximal incision in the posterior MIPO technique.

      Materials and methods

      Sample

      All MRI scans from our institutional database were retrospectively reviewed to analyze the location of the axillary and RNs. The main MRI indication was shoulder pain. We included scans showing the entire humerus and acromion of skeletally mature patients. We excluded scans of poor quality to identify major peripheral nerves and studies lacking complete images necessary to cross-reference nerve location, artifacts, and traumatic, tumorous, or infectious bone pathologies. Ultimately, 52 MRI scans were included. The mean age was 50.6 years (standard deviation, 12.1; range, 28-84 years), with 37 female and 15 male patients. The 59.6% were right handed (Table I).
      Table ISample data.
      Age (y)Humeral length (mm)
      Mean50.6273.2
      SD12.117.2
      CV (%)23.96.3
      Min28234.7
      Max84312.5
      95% CI47.2-53.9268.4-278.0
      SW P value.6870.7765
      SD, standard deviation; CV, coefficient of variation; Min, minimum; Max, maximum; CI, confidence interval; SW, Shapiro-Wilk.

      MRI data acquisition and processing

      The patients were placed in a supine anatomic position (Gantry tilt 0°) with elbow in full extension and scanned in a Philips Achieva 1.5T MRI machine (Philips Healthcare, Best, the Netherlands). MRI sequences included diffusion-weighted images with fat saturation and T1- and T2-weighted images in the axial, sagittal, and coronal planes (slice thickness 3 mm). The scans were analyzed with RadiAnt DICOM Viewer 2020.1.1 (Medixant, Poznan, Poland).

      Morphometric measurements

      Two independent observers (fellowship-trained shoulder and elbow surgeon and a shoulder and elbow fellow) measured the HL, the location of the axillary nerve (AN) in relation to a surgical anatomic landmark (PLAA) and a fluoroscopic landmark (acromioclavicular axis [ACA]), and the location of the RN in relation to a surgical anatomical landmark (PLAA) and a fluoroscopic landmark (transepicondylar axis [TEA]). These measurements were made in T1-weighted MRI coronal slices with sagittal and axial images to cross-reference nerve location. For reproducibility evaluation, all MRIs were remeasured 4 weeks later.
      The HL was defined as the straight line distance between PLAA and the lateral epicondyle (2D HL). The number of coronal imaging slices between these 2 points was recorded for later calculation of “3D HL” (Fig. 1).
      Figure thumbnail gr1
      Figure 1Humeral length. Humeral length (HL) was defined as the straight line distance between posterolateral angle of the acromion (PLAA) and the lateral epicondyle (LE). (A) PLAA was defined as the most posterior, inferior, and lateral point of the acromion, corroborated by the change in insertion of the deltoid muscle fibers. (B) LE was defined as the most prominent point (superior and lateral) of the LE on coronal images. (C) The PLAA point was copied and then pasted in the coronal slice of the LE (superior green arrow). The straight line distance between these 2 points was recorded as “2D HL.” The number of coronal imaging slices between these 2 points was recorded for later calculation as “3D HL.”
      The AN location was defined as the straight line distance between the PLAA and the most inferior branch of the AN at the humerus midpoint. The number of coronal imaging slices between these 2 points was recorded for later calculation of “3D AN.” ACA was defined as the straight horizontal line passing through the inferior aspect of the clavicle to the inferolateral point of the acromion. The perpendicular line distance between the AN to the ACA was recorded (Fig. 2).
      Figure thumbnail gr2
      Figure 2Axillary nerve. Axillary nerve (AN) location was defined as the straight line distance between the posterolateral angle of the acromion (PLAA) and the most inferior branch of the AN at the humerus midpoint. (A) Inferior branch of the AN at the posterior cortex of humerus. (B) The PLAA point was copied and then pasted in the coronal slice of the AN (superior green arrow). The straight line distance between these 2 points was recorded as “2D AN.” The number of coronal imaging slices between these 2 points was recorded for later calculation as “3D AN.” The formula to calculate the position of the AN from the PLAA in mm is 24.676-0.147 × age (years) + 0.146 × HL (mm). (C) Acromioclavicular axis (ACA) was defined as the straight horizontal line passing through the inferior aspect of the clavicle to the inferolateral point of the acromion. (D) The perpendicular line distance between the AN to the ACA was recorded. HL, humeral length.
      The RN location was defined as the straight line distance between the PLAA and 3 distinct positions: where it crossed the medial cortex of the humerus, where it crossed the middle point of the humerus, and where it crossed the lateral cortex of the humerus. The number of coronal imaging slices between these points was recorded for later calculation of “3D RN.” Also, the perpendicular line distance of the RN to the TEA was measured at these 3 locations. The TEA was defined as the longest distance between the most prominent points of the medial and lateral epicondyles on coronal images (Fig. 3).
      Figure thumbnail gr3
      Figure 3Radial nerve. Radial nerve (RN) location was defined as the straight line distance between the posterolateral angle of the acromion (PLAA) (A) and 3 distinct positions: where it crossed the medial cortex of the humerus, where it crossed the middle point of the posterior cortex of the humerus, and where it crossed the lateral cortex of the humerus. (C) The straight line distance between these points was recorded as “medial radial nerve location,” “middle radial nerve location,” and “lateral radial nerve location,” respectively. These measurements were made in T1-weighted MRI coronal slices with sagittal or axial images to cross-reference nerve location (D). The number of coronal imaging slices between these points was recorded for later calculation of “3D RN.” Also, the perpendicular line distance of the RN to the transepicondylar axis (TEA) was measured at these 3 locations (E). The TEA was defined as the largest distance between the most prominent points of the medial and lateral epicondyles on coronal images (B). MRI, magnetic resonance image; TEA, transepicondylar axis.

      Calculation of 3D distance

      Osseous anatomic landmarks and the location of a peripheral nerve were infrequently in the same coronal slice. Therefore, to determine the 3D distance between 2 points on an MRI, we used trigonometrical correction (Pythagoras theorem).
      • O'Shea R.
      • Panwar J.
      • Chu Kwan W.
      • Stimec J.
      • Camp M.W.
      • Gargan M.
      Establishing safe zones to avoid nerve injury in the approach to the humerus in pediatric patients: a magnetic resonance imaging study.

      Statistical analysis

      The results were analyzed with Stata BE17. The data presented normal distribution (Shapiro-Wilk test) and were presented as mean and standard deviation. The coefficient of variation (CV) was calculated. Differences between measurements were evaluated with a univariate analysis (Student’s t test) and a multivariate analysis (linear regression). P values < .05 were considered statistically significant. The intra- and inter-observer correlations were evaluated with the intraclass correlation coefficient (ICC). The study was approved by the ethics committee of our institution.

      Results

      The mean value of the HL was 273.2 ± 17.2 mm, and significant differences were found between females and males (267.8 ± 15.3 mm vs. 286.5 ± 14.5 mm; P = .0002; Table I). The location of the AN from the PLAA was 54.9 ± 6.4 mm (20.1 ± 2.0% HL; Table II), and from the ACA, it was 63.2 ± 6.1 mm (23.2 ± 2.2% HL; Table III). No significant differences were found between 2D and 3D measurements (Table III).
      Table IIThe location of the axillary nerve from the posterolateral angle of the acromion.
      2D (mm)3D (mm)HL (%)
      Mean54.956.320.1
      SD6.46.12.0
      CV (%)11.710.910
      Min41.844.114.7
      Max69.269.125.3
      95% CI53.1-56.754.5-58.014.7-25.3
      SW P value.6938.2297.6382
      Two-tailed P value.2749
      2D, two dimensional; 3D, three dimensional; HL, humeral length; SD, standard deviation; CV, coefficient of variation; Min, minimum; Max, maximum; CI, confidence interval; SW, Shapiro-Wilk.
      Table IIIThe location of the axillary nerve from the acromioclavicular axis.
      2D (mm)HL (%)
      Mean63.223.2
      SD6.12.2
      CV (%)9.79.5
      Min52.817.8
      Max79.528.4
      95% CI61.5-64.922.6-23.8
      SW P value.2977.6553
      2D, two dimensional; HL, humeral length; SD, standard deviation; CV, coefficient of variation; Min, minimum; Max, maximum; CI, confidence interval; SW, Shapiro-Wilk.
      The location of the RN from the PLAA was 100.2 ± 17.1 mm (36.6 ± 5.2% HL) at the medial humerus border, 118.0 ± 21.5 mm (43.1 ± 6.7% HL) at the middle humerus, and 146.0 ± 24.4 mm (53.6 ± 7.7% HL) at the lateral humerus border (Table IV, Fig. 4). The path of the RN attached to the humerus was 45.8 ± 14.6 mm (range, 14.9-80.1 mm). No significant differences were found between 2D and 3D measurements (Table IV).
      Table IVThe location of the radial nerve from the posterolateral angle of the acromion.
      Medial radial nerve locationMiddle radial nerve locationLateral radial nerve location
      2D (mm)3D (mm)HL (%)2D (mm)3D (mm)HL (%)2D (mm)3D (mm)HL (%)
      Mean100.2101.036.6118.0118.743.1146.0146.553.6
      SD17.116.95.221.521.36.724.424.37.7
      CV (%)17.116.714.118.217.915.716.716.614.4
      Min60.262.325.671.473.230.479.681.234.6
      Max151.2151.649.6180.3180.659.2212.4212.769.8
      95% CI95.5-105.096.3-105.735.2-38.0112.0-124.0112.8-125.041.2-45.0139.2-152.8139.8-153.351.4-55.7
      SW P value.6273.6108.9008.3780.3705.3728.8510.8586.3126
      Two-tailed P value.8107.8703.9094
      2D, two dimensional; 3D, three dimensional; HL, humeral length; SD, standard deviation; CV, coefficient of variation; Min, minimum; Max, maximum; CI, confidence interval; SW, Shapiro-Wilk.
      Figure thumbnail gr4
      Figure 4Radial nerve location. The location of the radial nerve (RN) from the posterolateral angle of the acromion (PLAA) was 100.2 ± 17.1 mm (36.6 ± 5.2% humeral length [HL]) at the medial humerus border, 118.0 ± 21.5 mm (43.1 ± 6.7% HL) at the middle humerus, and 146.0 ± 24.4 mm (53.6 ± 7.7% HL) at the lateral humerus border. The formula to calculate the position of the RN from the PLAA in mm is 0.611 × HL (mm) − 62.165. (A) Boxplot distribution of location of RN from PLAA (mm). (B) Boxplot distribution of location of RN from PLAA (% HL). (C) Boxplot distribution of location of RN from transepicondylar axis (TEA).
      The location of the RN from the TEA was 175.4 ± 15.6 mm (64.3 ± 5.6% HL) at the medial humerus border, 156.0 ± 19.0 mm (57.2 ± 6.8% HL) at the middle humerus, and 127.4 ± 21.2 mm (46.7 ± 7.8% HL) at the lateral humerus border (Table V). The location of the AN from the PLAA and from the ACA showed statistically significant differences between males and females (Table VI). The RN only showed statistically significant differences between males and females in relation to the PLAA (Table VI). When adjusting for the HL, statistically significant differences were only observed according to gender in the location of the AN in relation to the acromion (Table VI).
      Table VThe location of the radial nerve from the transepicondylar axis.
      Medial radial nerve locationMiddle radial nerve locationLateral radial nerve location
      2D (mm)HL (%)2D (mm)HL (%)2D (mm)HL (%)
      Mean175.464.3156.057.2127.446.7
      SD15.65.619.06.821.27.8
      CV (%)8.98.712.212.016.616.7
      Min131.850.7108.341.191.230.4
      Max213.877.5196.070.8177.166.5
      95% CI171.0-179.755.3-59.1150.7-161.355.3-59.1121.5-133.344.6-48.9
      SW P value.2466.5085.7856.3225.5738.9341
      2D, two dimensional; HL, humeral length; SD, standard deviation; CV, coefficient of variation; Min, minimum; Max, maximum; CI, confidence interval; SW, Shapiro-Wilk.
      Table VILocation of axillary and radial nerves by genre.
      AN from the PLAARN from the PLAA (MRNL)AN from the ACARN from TEA (MRNL)
      FemaleMaleFemaleMaleFemaleMaleFemaleMale
      Mean (mm)53.259.297.2107.560.869.1173.6179.8
      SD6.15.217.713.44.85.214.418.0
      95% CI51.1-55.256.4-62.191.3-103.1100.1-114.959.2-62.466.2-71.9168.8-178.4169.9-189.8
      SW P value.6448.9557.2892.2845.2764.9928.3181.6810
      Two-tailed P value.0014
      Statistically significant difference between female and male.
      .0481
      Statistically significant difference between female and male.
      .0000
      Statistically significant difference between female and male.
      .1923
      HL (%)19.920.736.237.622.824.264.962.7
      SD2.11.85.44.62.02.35.84.8
      95% CI19.2-20.519.7-21.734.4-38.035.0-40.222.1-23.422.9-25.463.0-66.960.1-65.1
      SW P value.3713.2623.9376.4699.6392.9486.9553.9428
      Two-tailed P value.1756.3791.032∗.2005
      AN, axillary nerve; PLAA, posterolateral angle of the acromion; RN, radial nerve; MRNL, medial radial nerve location; ACA, acromioclavicular axis; TEA, transepicondylar axis; SD, standard deviation; CI, confidence interval; SW, Shapiro-Wilk; HL, humeral length.
      Statistically significant difference between female and male.
      When adjusting for the location of both nerves through a linear regression model, considering age, gender, and HL, a significant correlation of both nerves with the HL was observed, independent of gender. In the AN, a correlation with age was also observed (Table VII). Regarding the intra- and inter-observer correlations, the mean intraobserver ICC was 0.91 ± 0.09, and the mean interobserver ICC was 0.81 ± 0.26 for all measures (Table VIII).
      Table VIILinear regression for axillary and radial nerve location.
      VariableCoefficientSEP value
      Axillary nerve from the PLAA
       Age (y)−0.1470.061.019
       Genre (female)−3.2111.839.087
       Humeral length (mm)0.1460.049.004
       Constant24.67614.414.093
       F P value.0000--
      Radial nerve from the PLAA (MRNL)
       Age (y)−0.1050.163.522
       Genre (female)1.1894.948.811
       Humeral length (mm)0.6110.132.000
       Constant−62.16538.787.116
       F P value.0001--
      PLAA, posterolateral angle of the acromion; SE, standard error; MRNL, medial radial nerve location.
      Table VIIIIntraobserver−interobserver correlation.
      VariableICC intraICC inter
      2D humeral length0.9800.982
      2D axillary nerve location0.9760.934
      ACA—axillary nerve0.7790.838
      2D medial radial nerve location0.9790.974
      2D middle radial nerve location0.9830.913
      2D lateral radial nerve location0.8330.942
      TEA—medial radial nerve location0.7330.405
      TEA—middle radial nerve location0.9500.960
      TEA—lateral radial nerve location0.9750.303
      ICC, intraclass correlation coefficient; Intra, intraobserver; Inter, interobserver; ACA, acromioclavicular axis; TEA, transepicondylar axis.

      Discussion

      The main finding of our study is that the location of the axillary and RN in relation to the posterior aspect of the humerus is related to the HL and can be used to predictably define the safe zones to avoid nerve injury in the proximal incision of posterior MIPO for humerus fractures.
      Regarding the location of the AN, both references presented CVs around 10% (range, 9.5-11.7). Considering the excellent intra- and inter-observer correlations, the low CV is attributed to minimal anatomic variability. Furthermore, because no differences were found between the 2D and 3D measurements, the use of the PLAA would be adequate because it is a reliable palpable anatomic landmark.
      • Apivatthakakul T.
      • Patiyasikan S.
      • Luevitoonvechkit S.
      Danger zone for locking screw placement in minimally invasive plate osteosynthesis (MIPO) of humeral shaft fractures: a cadaveric study.
      Several studies have used the HL as a reference to describe the proportional distance between neurovascular structures and osseous landmarks.
      • Apivatthakakul T.
      • Patiyasikan S.
      • Luevitoonvechkit S.
      Danger zone for locking screw placement in minimally invasive plate osteosynthesis (MIPO) of humeral shaft fractures: a cadaveric study.
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      • Uslu M.
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      • Comert A.
      • Tekdemir I.
      • Cift H.
      Is there a safe area for the axillary nerve in the deltoid muscle? A cadaveric study.
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      • Fleming P.
      • Lenehan B.
      • Sankar R.
      • Folan-Curran J.
      • Curtin W.
      One-third, two-thirds: relationship of the radial nerve to the lateral intermuscular septum in the arm.
      ,
      • O'Shea R.
      • Panwar J.
      • Chu Kwan W.
      • Stimec J.
      • Camp M.W.
      • Gargan M.
      Establishing safe zones to avoid nerve injury in the approach to the humerus in pediatric patients: a magnetic resonance imaging study.
      In our study, when adjusting for the HL, the anatomic variability of AN is maintained, probably associated with a more proximal location and a shorter and perpendicular trajectory.
      Jiamton et al
      • Jiamton C.
      • Ratreprasatsuk N.
      • Jarayabhand R.
      • Kritsaneephaiboon A.
      • Apivatthakakul T.
      The safety and feasibility of minimal invasive plate osteosynthesis (MIPO) of the posterior aspect of the humerus: a cadaveric study.
      found an average distance of 47.9 mm from the PLAA (17.5% HL), similar to our findings. The HL of their sample (268.6 mm) was similar to ours (273.2 mm).
      In relation to the location of the RN, the PLAA reference presented CVs around 16.4% (range, 14.1-18.2), which is likely associated with higher anatomic variability and a longer path. In contrast, the TEA reference presented a variability around 12.5% (range, 8.7-16.7). The variability decreased when adjusting for the HL, with the “medial RN” being the least variable in both measurements. Although no differences were found between the 2D and 3D measurements, the fluoroscopic reference is the most reliable.
      Jiamton et al
      • Jiamton C.
      • Ratreprasatsuk N.
      • Jarayabhand R.
      • Kritsaneephaiboon A.
      • Apivatthakakul T.
      The safety and feasibility of minimal invasive plate osteosynthesis (MIPO) of the posterior aspect of the humerus: a cadaveric study.
      found an average distance of 104.7 mm between the PLAA and the location where the RN crossed the medial border of the humerus (39% HL), similar to our findings. Yingling et al
      • Yingling J.M.
      • Yoon R.S.
      • Liporace F.A.
      Reliable method of radial and ulnar nerve identification during the posterior approach to the humerus: a clinical and cadaveric correlation study.
      also evaluated the location of the RN in relation to the lateral epicondyle in a posterior approach; however, they identified the nerve along the lateral border of the triceps before diving deep into the brachioradialis for an average of 75.9 mm, which is a shorter distance than that found in our study (127.4 ± 21.2 mm). This discrepancy is likely associated with the fact that the RN was measured at the point where it crosses the intermuscular septum. Our measurements were made where the RN crosses the posterior cortex, somewhat more proximally.
      Various anatomic studies described that the RN pierces the lateral intermuscular septum at an average range of 102-125 mm proximal to the lateral epicondyle.
      • Cox C.L.
      • Riherd D.
      • Tubbs R.S.
      • Bradley E.
      • Lee D.H.
      Predicting radial nerve location using palpable landmarks.
      ,
      • Kamineni S.
      • Ankem H.
      • Patten D.K.
      Anatomic relationship of the radial nerve to the elbow joint: clinical implications of safe pin placement.
      ,
      • Simone J.P.
      • Streubel P.N.
      • Sánchez-Sotelo J.
      • Steinmann S.P.
      • Adams J.E.
      Fingerbreadths rule in determining the safe zone of the radial nerve and posterior interosseous nerve for a lateral elbow approach: an anatomic study.
      However, in the posterior MIPO approach, the relevant position is the path of the RN in relation to the posterior cortex of the humerus. Furthermore, most of these studies evaluate the use of lateral approaches. Reported measurement differences may have been due to these different references chosen for the proximal measurement or to variability in the course of the RN in the distal aspect of the humerus.
      In all the measurements made, significant differences were found in relation to gender (Table VI). However, when adjusting for the HL, only the gender-based difference in the location of the RN, as evaluated by fluoroscopy, remained significant. When adjusting for age, gender, and HL, multivariate analysis demonstrated a gender-independent association of the location of both nerves with the HL. Furthermore, the position of the AN was inversely correlated with age, probably associated with deltoid muscle atrophy or elevation of the humeral head secondary to rotator cuff disease. Also, the HL is more relevant in the position of the RN than the AN, which is likely due to the most proximal location, trajectory, and anatomic variability.
      The anatomic relationship of the RN to the medial and lateral epicondyles and the TEA has been previously described.
      • Fleming P.
      • Lenehan B.
      • Sankar R.
      • Folan-Curran J.
      • Curtin W.
      One-third, two-thirds: relationship of the radial nerve to the lateral intermuscular septum in the arm.
      ,
      • O'Shea R.
      • Panwar J.
      • Chu Kwan W.
      • Stimec J.
      • Camp M.W.
      • Gargan M.
      Establishing safe zones to avoid nerve injury in the approach to the humerus in pediatric patients: a magnetic resonance imaging study.
      Despite the lower variability of RN measurements in relation to the TEA, it must be considered that patients present humerus fracture, so the position of this axis will vary. In patients with comminuted fractures, the use of the TEA is limited, so its practical usefulness is limited to simple fractures that can be reduced anatomically. Based on the above, we suggest the use of this value as a reference to confirm the measurements in relation to the PLAA.
      The proximal safe zone to avoid nerve injury in the posterior MIPO for humerus fractures is between the AN (proximal limit) and the location where the RN crosses the lateral border of the humerus (distal limit). These limits can help avoid damage of the AN with a long to proximal plate and help selectively dissect the RN to ensure its superficial position relative to the plate.
      Considering the PLAA reference, the proximal safe zone area would be between 54.9 mm of the AN (20.1% HL) and 146 mm of the RN (53.6% HL). However, identifying the AN is not necessary in most cases. If a long bridging plate that extends up into this area is required, the AN must be identified; however, management of the RN is key. Further considering that the trajectory of the RN in the humerus averages 45.8 mm, we prefer to use the reference of the medial border of the humerus and extend distally (lower CV). Radiographically, we can confirm this measurement in relation to the TEA.
      It should be considered that the minimum cutaneous incision needed for proximal incision is approximately 5 cm, but this window is flexible in relation to the abundant adipose pad in the area. In relation to the above, we recommend a 50-mm incision from the crossing of the medial edge of the humerus by the RN (100.2 mm), confirming the Gallucci recommendation (Fig. 5).
      • Gallucci G.L.
      • Boretto J.G.
      • Vujovich A.
      • Alfie V.A.
      • Donndorff A.
      • De Carli P.
      Posterior minimally invasive plate osteosynthesis for humeral shaft fractures.
      Figure thumbnail gr5
      Figure 5Proximal incision for posterior minimally invasive plate osteosynthesis. Considering our results, we recommend a 50-mm incision from the crossing of the medial edge of the humerus by the radial nerve (100.2 mm distal to the PLAA, posterolateral acromion angle) for a direct visualization and protection. PLAA, posterolateral angle of the acromion.
      Balam and Zahrany
      • Balam K.M.
      • Zahrany A.S.
      Posterior percutaneous plating of the humerus.
      demonstrated the safety of posterior MIPO for humerus fractures in 37 patients. Only 2 patients (5.4%) had postoperative RN palsy, but both recovered spontaneously. Gallucci et al
      • Gallucci G.L.
      • Boretto J.G.
      • Alfie V.A.
      • Donndorff A.
      • De Carli P.
      Posterior minimally invasive plate osteosynthesis (MIPO) of distal third humeral shaft fractures with segmental isolation of the radial nerve.
      reported that in a series of 21 patients with posterior MIPO, one patient developed an RN palsy that recovered 6 weeks after surgery.
      At the proximal incision, the RN lies between the lateral and long heads of the triceps along the profunda brachii vessels. Dissection should be meticulous to protect the nerve. The RN and the vessels should be lifted carefully from the bone, creating as little tension as possible to ensure that the RN will not be trapped under the plate during the submuscular tunneling and the plate insertion.
      • Jiamton C.
      • Ratreprasatsuk N.
      • Jarayabhand R.
      • Kritsaneephaiboon A.
      • Apivatthakakul T.
      The safety and feasibility of minimal invasive plate osteosynthesis (MIPO) of the posterior aspect of the humerus: a cadaveric study.
      A useful anatomic landmark is the Apex of Triceps Aponeurosis.
      • Arora S.
      • Goel N.
      • Cheema G.S.
      • Batra S.
      • Maini L.
      A method to localize the radial nerve using the 'apex of triceps aponeurosis' as a landmark.
      The RN crosses the humerus approximately 25 mm proximal to this landmark, so we must look at the most distal aspect of the proximal window for the Apex of Triceps Aponeurosis to identify the RN more accurately at the midpoint of the incision.
      Many previous anatomic studies have used cadavers. This limited sample reduced the accuracy of their results. In addition, formalin causes shrinkage and dehydration of soft tissue, which may alter anatomical measurements.
      • Park J.K.
      • Choi S.M.
      • Kang S.W.
      • Kim K.J.
      • Min K.T.
      Three-dimensional measurement of the course of the radial nerve at the posterior humeral shaft: an in vivo anatomical study.
      Moreover, disarticulation of the shoulder joint and dissection of the RN may have affected the course of the nerve.
      • Scolaro J.A.
      • Hsu J.E.
      • Svach D.J.
      • Mehta S.
      Plate selection for fixation of extra-articular distal humerus fractures: a biomechanical comparison of three different implants.
      Therefore, we conducted an in vivo anatomical study based on MRI that we believe overcomes these reported drawbacks. In addition, the study of the location of the nerves with MRI showed excellent intra- and inter-observer correlations, which allows us to attribute variability to anatomic characteristics.
      This study had several limitations, including humerus without fracture, its retrospective design, and the fact that it was performed at a single center. Nevertheless, the absence of fracture mainly affects measurements related to TEA because the indication for posterior MIPO includes those cases with fractures distal to the RN, so the measures from the acromion should not be affected; in any case, it is a consideration before its application to clinical practice. It would be advisable to analyze anatomic variations in cases of patients with humerus fracture.
      The retrospective nature of the study meant that we were unable to replicate the shoulder and elbow positioning during surgery. There are various anatomic studies with contradictory results in relation to this aspect. Artico et al demonstrated no major changes in the localization of the RN at the posterior aspect of the humerus with elbow position.
      • Artico M.
      • Telera S.
      • Tiengo C.
      • Stecco C.
      • Macchi V.
      • Porzionato A.
      • et al.
      Surgical anatomy of the radial nerve at the elbow.
      Hackl et al determined that with 90° of elbow flexion, the distance of the RN from the anterior edge of the capitellum changed, on average, by 3.6 mm.
      • Hackl M.
      • Lappen S.
      • Burkhart K.J.
      • Leschinger T.
      • Scaal M.
      • Müller L.P.
      • et al.
      Elbow positioning and joint insufflation substantially influence median and radial nerve locations.
      Recently, Chen et al noted that the RN excursion is doubled as the elbow is flexed from 0° to 90°.
      • Chen W.A.
      • Luo T.D.
      • Wigton M.D.
      • Li Z.
      Anatomical factors contributing to radial nerve excursion at the brachium: a cadaveric study.
      We believe that the elbow position had no influence on the position of the RN, as it crossed the posterior humerus cortex because it is tightly adhered.
      Also, the position of the shoulder and its influence on the location of the AN has been scarcely evaluated. Samart et al reported that the average distance from the lateral acromial edge to the AN at a shoulder abduction of 45° and 90° were 57.1 and 52.9 mm, respectively.
      • Samart S.
      • Apivatgaroon A.
      • Lakchayapakorn K.
      • Chemchujit B.
      The correlation between acromion-axillary nerve distance and upper arm length; a cadaveric study.
      Bailie et al reported that the distance between the AN and the PLAA averaged 65 mm and decreased by an average of 14 mm (22%) with the shoulder.
      • Bailie D.S.
      • Moseley B.
      • Lowe W.R.
      Surgical anatomy of the posterior shoulder: effects of arm position and anterior-inferior capsular shift.
      Our results must be corroborated with studies that evaluated the impact of the position of the shoulder and elbow.

      Conclusion

      The main finding of our study is that the location of the axillary and RN in relation to the posterior aspect of the humerus is related to the HL and can be used to predictably define the safe zones to avoid nerve injury in the proximal incision of posterior MIPO for humerus fractures. Considering the PLAA reference, the proximal safe zone area would be between 54.9 mm of the AN (20.1% HL) and 146 mm of the RN (53.6% HL).

      Acknowledgments

      The authors acknowledge their families for their unconditional support.

      Disclaimers

      Funding: No funding was disclosed by the authors.
      Conflicts of interest: R.L. and F.S. have given educational talks financed by Zimmer-Biomet related to the subject of this article. The other authors, their immediate families, and any research foundation with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.

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