</div>

2.3 Modeling of Surgical Guide Template

A rectangular mold was developed as a drill guide template using Mimics software. Then, the surgical guide was fitted on the first implanted humerus bone (Figure 5). The surgical guide needs to accurately seat on the distal of the humerus bone. The humerus bone was cut for further realization of the method. The modeled implant was inserted into the humerus bone based on the flexion-extension axis. A hole was made based on the position of the implant stem in the bone. In order to do it, a bar (with a similar dimension to the implant stem) was precisely fitted on the stem of the implant stem (Figure 6). A hole created by the bar had a diameter of 5 mm. The surgical guide was designed based on the shape of the distal region. Then, a hole was made in the surgical guide based on the direction and position of the implant stem. Finally, to lead the drill in the right direction, a drill guide was designed based on the position and direction of the hole in the bone. The same procedure was performed on all humerus bones.

(a) (b) (c) (d)

2.4 Fabrication and Metal Sleeve Preparation

Surgical guides and humerus bones were fabricated, using a 3D printer (Dimension elite, Stratasys, USA) technique (Figure 7). CAD files of humerus bones and the surgical guides, in STL format, were transferred to the 3D printer. A metallic sleeve was fixed inside the hole of the surgical guide that guides the drill to accommodate the passage of the drill precisely. A standard tube (Ekinciler Iron and Steel IND. INC, Istanbul, Turkey) with an internal diameter of 5.2 mm and an external diameter of 5.5 mm was provided for the metal sleeve. As the internal diameter of the sleeve should be 0.2 mm bigger than the diameter of the drill [29, 30], a sleeve with a 5.2 mm internal diameter was used for a 5 mm surgical drill. The tube was cut into 5 mm length, using a saw. A super glue (UHU, UHU GmbH & Co. KG, Germany) was used to fix the metallic sleeve firmly inside the drill guide.

(1) (b) (c)

2        The position of the holes was measured in the Mimics software to validate the accuracy of the method. A power drill with a 5.0 diameter drill bit was used to make a hole in the bones (Figure 8). A F-clamp was used to fix the bones, then, the surgical guides were set on the bones precisely. Since the surgical guides were designed based on the shape of the bones, they were mounted on the bone smoothly and without any movement to the sides. Consequently, the bones were drilled using a drill machine. A caliper was used to measure the position of the planned and achieved holes in the bones. The positions of the tip and base of the bone are as follows:

-Base-distance: Diversity in distances at the implanted bone shoulder in the direction of medial/lateral and posterior/anterior.

-Tip-distance: Diversity in distances at the implanted bone apex in the direction of medial/lateral and posterior/anterior. 

3 RESULTS

Figure 9 and Figure 10 show the achieved and planned positions of the hole at the bone apex in the medial/lateral direction. On one side, the Mimics software package was used to make a hole and on the other side, similar holes were made experimentally, using the surgical guide template. Virtually, the positions of the holes were measured at 13.88 mm, 18.45 mm, 17.37 mm, and 18.77 for Sample 1, 2, 3, and 4 in the medial/lateral direction, respectively. Experimentally, the positions of the holes were measured at 13.63 mm, 18.07 mm, 16.91 mm, and 18.24 for Samples 1, 2, 3, and 4, respectively, in the same direction.

Sample 1 Sample 2 Sample 3 Sample 4

Figure 11 and Figure 12 present the achieved and planned positions of the hole at the bone apex in the posterior/anterior direction. Virtually, the positions of the holes were measured at 10.70 mm (Sample 1), 14.91 mm (Sample 2), 9.88 mm (Sample 3), and 14.04 (Sample 4) in the posterior/anterior direction. Experimentally, the positions of the holes were measured at 10.42 mm, 14.32 mm, 9.31 mm, and 13.46 for Sample 1, 2, 3, and 4 respectively. The deviations (errors) between the achieved and planned holes at the bone apex were seen at 0.28 mm, 0.59 mm, 0.57 mm, and 0.58 mm for samples 1, 2, 3, and 4 respectively in the posterior/anterior direction. However, the deviations in the medial/lateral direction were 0.25 mm, 0.38 mm, 0.46 mm, and 0.53 mm for samples 1, 2, 3, and 4 respectively (Figure 13).

Sample 1 Sample 2 Sample 3 Sample 4

The achieved and planned positions of the holes at the base of the bone in the medial/lateral direction are shown in Figure 14 and Figure 15. The positions of the holes (experimentally) were measured at 7.81 mm, 7.04 mm, 5.37 mm, and 4.50 mm for Samples 1, 2, 3, and 4 respectively. Virtually, the positions of the holes for samples 1, 2, 3, and 4 were measured at 8.10 mm, 6.62 mm, 5.95 mm, and 4.82 mm in the same direction, respectively. 

Sample 1 Sample 2 Sample 3 Sample 4

The achieved and planned positions of the holes at the base of the bone in the posterior/anterior direction are shown in Figure 16 and Figure 17. Virtually, the positions of the holes for Samples 1, 2, 3, and 4 were measured at 6.27 mm, 8.11 mm, 5.74 mm, and 6.16 mm, respectively. Experimentally, the positions of the holes were measured at 6.68 mm, 8.37 mm, 6.25 mm, and 5.60 mm for Samples 1, 2, 3, and 4, respectively. In the posterior/anterior direction, the deviations at the base of bones were 0.41 mm (Sample 1), 0.26 mm (Sample 2), 0.51 mm (Sample 3), and 0.56 mm (Sample 4) (Figure 18). However, the deviations were 0.29 mm, 0.42 mm, 0.58 mm, and 0.32 mm for Samples 1, 2, 3, and 4 respectively, in the medial/lateral direction.

Sample 1 Sample 2 Sample 3 Sample 4

4 Discussion

There are no good results for the elbow joint replacements when compared with the knee and hip replacements. Early loosening of joint arthroplasties, instability, limited range of motion, and dislocation in patients with articulated fixators, subluxation, and excessive polyethylene wear are the results of incorrect selection of the flexion-extension axis during surgery [1, 2, 31]. While various mechanisms of implant loosening, as the main causes of elbow implant failure, have been recommended, implant malpositioning might probably be the important underlying reason. This axis is related to the alignment tools during surgery. After surgery, the kinematics of the elbow axis is altered; hence, the muscle moment arms and the arm motion pattern also change. This enhances the load at the articular bearing, which causes loosening and implant wear [32, 33]. Proper selection of the flexion-extension axis results in accurate implant placement. A very tiny error on the distal humerus width (only a few millimeters in the axis endpoints selection) can change several degrees in orientation. Previous studies had investigated the importance of implant alignment with respect to load transfer and kinematics [34, 35]. The position of the stem in the canal has a vital role in the survivability of the implant. Optimum implant positioning and technique used for its alignment are vital parameters to improve clinical results. A study showed that a gradual increase in prosthetic survival is possible through the development of surgical techniques [36]. The orthopedic CAD/CAM surgery is becoming more popular with a concentration on the spine, hip, and knee. However, the elbow joint suffers from a lack of knowledge [37, 38]. Hence, in this study, a technique was used to model the surgical guide template to identify the flexion-extension axis, position, and orientation of stem inside the humerus canal. The used technique permitted the conversion of a radiographic guide to a surgical guide template.

Visual cues and jigs are the current surgical techniques used to estimate the elbow’s flexion-extension axis for TEA [9, 16, 17]. The surgeons extend this line (during surgery) visually to make an intersection between the lateral and medial surfaces of the distal humerus, close to the epicondyles. Morrey et al. (2000) presented this technique in an external dynamic joint distractor [11]. However, Brownhill et al. (2006) proved that this technique can lead to prominent errors for the flexion-extension axis. Nevertheless, the use of cues and jigs may lead to flexion-extension axis identification errors upwards of 10° [19, 39]. 

Recently, CAD/CAM has been employed to enhance the precision of determining the elbow’s flexion-extension axis. The appropriate implant position and mechanical limb alignment could be more accurately produced with the computer-assisted system compared with the conventional and traditional techniques. The establishment of a preoperative plan, as well as diagnosis, could be more reliable when the 3D medical image of the anatomy was obtained. Surgeons are allowed to use navigated guidance by registering the preoperative data to the patient while referencing the benchmarks on the preoperative plan that is not accessible in the surgical exposure. This can assist the surgeons to create a treatment plan prior to implant placement [40, 41]. The recent progress in medical imaging and computer field aids surgeons to use the newest equipment, such as a CT scanner. The most distinguished benefit of the CT scan method is the higher level of precision than other methods. Its proficiency has been mentioned in previous studies [42]. There are various applications of CAD in biomedical engineering such as tissue engineering, implant design, medical device design, and preoperative simulations.

In this study, 3D surgical site reconstruction was achieved, using CT data. This permitted the fabrication of a surgical guide using RP and the virtual implant planning via restorative considerations. The flexion-extension line was expanded to intersect the lateral and medial surfaces of the distal humerus, using Materialise Mimics software package. The usual method was used to match the planned and achieved positions by measuring the transfer precision of the computer-based 3D plan. The results showed that the deviations between the achieved and planned holes at the bone apex were 0.28 mm, 0.59 mm, 0.57 mm, and 0.58 mm for samples 1, 2, 3, and 4 respectively in the posterior/anterior direction. However, the deviations in the medial/lateral direction were 0.25 mm, 0.38 mm, 0.46 mm, and 0.53 mm for Sample 1, 2, 3, and 4, respectively. At the base of the bone, the samples 1, 2, 3, and 4 were experienced deviations at 0.41 mm, 0.26 mm, 0.51 mm, and 0.56 mm respectively in the posterior/anterior direction. Furthermore, the deviations were 0.29 mm, 0.42 mm, 0.58 mm, and 0.32 mm for Sample 1, 2, 3, and 4, respectively, in the medial/lateral direction.

Brownhill et al. applied real-time computer feedback to position the humeral component in 5 cadaveric extremities. The overall positioning errors were 1.7±2.1 mm (superior), 1.5±1.2 mm (anterior), and 1.2±1.4 mm (medical) relative to the native articulation [38]. McDoland et al. (2007) evaluated the conventional paired-point registration method against the surface-based registration technique. The conventional registration method showed an alignment error at 1.9 +/- 1.0 mm, whereas the surface-based registration method presented an error at 0.8 +/- 0.3 mm [43]. In another study, McDoland et al. (2011) validated the precision of a computer-aided implant placement technique. The study was carried out for both the modified and regular humeral components. The average alignment error of the regular stem was 1.9 ± 1.1 mm, while the reduced stem showed an averaged error of 1.3 ± 0.5 mm in translation [44]. Furthermore, McDoland et al. (2010) investigated the humeral component implant alignment, using a CT-based preoperative plan measured to landmarks on the distal humerus. The alignment was carried out under two categories, namely intact humerus and humerus without articular landmarks. The navigated implant alignment was 1.2 +/- 0.3 mm for the intact bone and 1.1 +/- 0.5 mm for the bone loss. For the non-navigated alignment method, the alignment was 3.0 +/- 1.6 mm for the bone loss category and 3.1 +/- 1.3 mm for the intact scenario [45]. Deluce et al. (2018) used an intra-operative surface-based registration technique for a radial head implant. The positioning errors were 0.8 ± 0.5 in the medial-lateral direction and 0.8 ± 0.6 in the anterior-posterior direction. A registration error of less than 1.0 mm was also observed [46]. In this study, the deviation (error) between the achieved and planned positions was less than 0.50 mm. The value was less than the reported implant placement, using a computer-aided surgical system. The introduced method prepared a radiographic guide that can be converted into a surgical guide template. In addition to this, the use of multiple guides is not necessary for the drilling sequence. This surgical guide template can be used in a wide range of implant systems.

Generally, drastic destruction of the elbow happens in the replacement of an elbow joint, which is especially visible in patients with RA. The more proximal location of an implant in the affected elbows is a result of the deficient distal humerus bone stock. The visual assessment of anatomic landmarks is more complicated in elbows with severe bone loss, such as fractures, tumors, and end-stage RA. This causes more error in the estimation of the flexion-extension axis and eventually, the alignment targets and precise identification of landmarks tend to vary under non-ideal conditions encountered during surgery. Errors in orienting landmarks can increase throughout the operation when the cutting guides or jigs are referred from these anatomic reference points. However, in the current study, CT data were used for the estimation of the flexion-extension axis before surgery. The flexion-extension axis was determined after carefully surveying the bone. Therefore, proper alignment in the introduced technique is more likely to happen than when jig and cues are used during surgery.  

In the present study, a drill guide was considered for the surgical guide. A metallic guide was relatively matched to the drill and implant diameter to improve the precision of the surgical guide. The main body of the surgical guide was suitably fitted on the bone to ensure the stability of the template during surgery. Holes were made based on the planned position of the stem. A stainless-steel tube was truly fixed within the hole to protect the ABS plastic against the sharpness of the drill. When performing surgery, a tube size that is 0.2 mm wider than the drill can be used to prevent the drill from contacting the stainless steel tube [47]. In this study, the height of the drill guide was not evaluated as it is not a pivotal parameter for the precision of implant placement [21]. Surgical guides without accurate drill guides are used as a proximate guide only. Therefore, the implant may be placed in a different position compared to the planned position. 

Several studies have investigated the precision of the CAS method. Van Steenberghe et al. (2002) investigated the CAD/CAM surgical guide accuracy in cadavers, whereas Vrielinck et al. (2003) carried out the same study on human subjects [48, 49]. Gschwend et al. (2000) used a template to insert a guiding Steinmann pin inside the humerus bone to make a hole preoperatively [50], while Donald et al. evaluated a different way of humeral stem positioning. The computer-assisted technique for TEA was proven to improve implant alignment compared to the conventional methods [18]. Stacpoole et al. studied the effect of radial head implant position on joint kinematics and tracking using a computer-aided implant placement procedure [51]. Recently, surgeons have attained a modified tool for the alignment of the hip, shoulder, and knee. Although it demonstrated much promise, there has been quite limited application of this technology to the elbow, and has, thus far, been limited to laboratory studies only. Since elbow replacements may not be used by most surgeons, the navigation system may be preferred over higher-volume arthroplasties such as the hip and knee. This may result in a decrease in wear for linked implants and reduced instability for unlinked devices. However, clinical studies should confirm these hypotheses.

Clinical studies have shown a higher rate of loosening for the humeral component than the ulnar component. Excessive loading of the implant stem is caused by malpositioning of the humeral component [52, 53]. The humeral stem restricts modularity in the orientation and situation of the implant articulation axis. Previous studies showed that the kinematics change of elbow replacement was due to the malrotation of the humeral component. This malposition would cause elbow wear, laxity, and loosening [33, 54]. Consequently, in this study, the surgical guide was designed only for the humeral stem. 

To the authors’ knowledge, this study is the first to use such a technique for humeral implant positioning. Even though the errors in implant positioning were greater than 1 mm, the results of this study showed improvement in terms of surgical errors. The introduced method possesses a lot of advantages for TEA. Firstly, the surgeon is able to select the size and placement of the hole based on the unique morphology of the humerus bone before surgery. The second advantage is the preoperatively prepared drill guide can be used to make the hole in proper direction and angulation. Thirdly, it does not require complex equipment in the operating room. Last but not least, it is easy to make and use. Furthermore, it is possible to use different drill sizes and it does need to be adjusted by a jig. This study recommends the use of a 3D planned template as a surgical guide as it is a reliable technique for implant placement during surgery.

ACKNOWLEDGMENT

We acknowledge Global College of Engineering and Technology (Oman) for supporting this study.

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