This study evaluated the potential of PDS foils with different thicknesses for orbital floor reconstruction through the use of 22 cadaver orbits. The advantages associated with performing orbital reconstruction using resorbable alloplastic materials (such as PDS foils) include the immediate availability of material, no morbidity at a donor site, and the capacity of the material to support the orbital contents via the formation of an autologous soft tissue scar as it degrades [10, 11]. Compared to thicker PDS foil material, the use of thinner PDS foil material can reduce postoperative complications [19, 21], such as the extent of foreign body reactions, and thinner materials have a higher bending capacity due to their lower flexural stiffness [26], which theoretically corresponds more closely to orbital funnel geometry.

Regardless of its capacity to reconstruct the geometric specifications of the native orbit, the biomechanical stability of PDS foil material in the presence of extended orbital floor defects continues to be debated. Assuming that PDS foil material serves as a temporary, non-load-bearing barrier between the orbital and sinus compartments and not as a load-bearing retention [11], some scholars are concerned that there is inadequate support for the orbital contents after the resorption of the foil material, which increases the risk of developing diplopia and late enophthalmos when the defect size exceeds the critical threshold of 250–300 mm2 [13, 16,17,18]. However, the use of PDS foil material in the reconstruction of defects with a median size of 432.0 mm2 [5] and isolated orbital floor defects of up to 1019.0 mm2 [19] without the development of secondary complications attests to the need for an in-depth discussion of the biomechanical principles that apply when reconstructing defects of various sizes.

The orbital content weight is estimated to be 30 g or 0.3 N [21, 23, 24]. With a median orbital floor defect size of 450.7 mm2 in this study, the repositioned orbital contents theoretically acted with a static mechanical force of 0.000666 MPa (N/mm2) on the inserted reconstruction material [20, 23]. The initial puncture strength of a 0.15 mm thick PDS foil was experimentally found to be 2.57 MPa, which corresponds to three times the puncture strength of the native orbital floor [20]. Hence, for a defect with a median area of 450.7 mm2 (as tested in this study), a 0.15 mm thick PDS foil would theoretically still have a load-bearing capacity of 0.01149 MPa eight weeks after insertion [21, 24]. Given this discrepancy between the actual force of the orbital contents and the puncture strength of the inserted foil material [21, 23, 24], it may be argued that the resorbable foil material in this case was not merely a separation barrier [11], as the potential load-bearing capacity of the PDS foils did not appear to be fully exhausted. These biomechanical findings indicate that the stability of both the 0.25 mm and 0.5 mm foils was sufficient for initially bridging the extended orbital floor defects created in the current study; conventional and pre-bent titanium meshes or patient-specific implants would also have been suitable [27,28,29].

An inherent limitation of this study, and other cadaveric studies, is that the orbital contents differed from those in real living tissues, demonstrating lower elasticity, inconstant bulbus pressure, postmortem ocular degeneration and hypotrophy, and an inability to generate negative pressure in the paranasal sinuses [21, 23], which cannot be fully countered by manual re-tensioning of the eyeball to 30 g. The influence of dynamic forces and time on the resorbable reconstruction material [20,21,22] was not considered in this study, as this work focused on comparing the reconstructive accuracy of two different PDS foil thicknesses for the repair of standardized orbital floor defects.

Orbital floor reconstruction must not only meet biomechanical needs but also ensure the accurate restoration of the orbital geometry. Since orbital volume and orbital height determine the bulbus position and ensure facial harmony, these parameters are used to clinically assess the sufficiency of the initial reconstructive geometry [4, 5, 16]. After trauma, both orbital volume and height increase, as the orbital contents herniates through the defect site into the maxillary sinus [25, 30,31,32]. Treatments for primary orbital floor fractures are generally designed to result in a true-to-original reconstruction of the orbital geometry [33], and the outcomes of such treatments have steadily improved with the introduction and development of patient-specific implants [29]. However, other studies have reported no difference in volumetric reconstructive accuracy at 12 weeks [28, 34] and six months [35] in patients with favorable and unfavorable clinical outcomes. Instead, volumetric tolerance [31] appears to depend primarily on the direction of the trend in volume deviation and the timing of orbital floor reconstruction. An increase in the orbital volume, due to herniation of orbital tissue after trauma [36, 37] or deformation of resorbable foil material after reconstruction of the orbital floor [32], has been reported to carry an increased risk for the development of enophthalmos. Whereas a decrease in the orbital volume (i.e., overcorrection of the orbital geometry) might reflect the need for secondary reconstructive procedures [38], which are required in 0.4% of cases after initial treatment of orbital blowout fractures [39] and are intended to camouflage soft tissue deficits [40] or compensate for a relapse in globe projection after correction of enophthalmos [41]. Overcorrection of the orbital geometry in the clinical context of primary treatment of orbital floor defects is largely underrepresented in the literature but is a known phenomenon in cadaveric studies [42, 43], to which the abovementioned limitations of cadaveric studies apply.

In this cadaveric study, overcorrection of the orbital height and volume were observed, regardless of the PDS foil material thickness. The vertical orbital height after orbital floor reconstruction is influenced by the bending behavior of the PDS foil material, which is initially determined by the force that the bulbus applies to the reconstruction material as a function of the orbital defect area [20, 23]. Compared to the initial bending of a 0.15 mm thick PDS foil [21, 24], the PDS 0.25 mm and PDS 0.5 mm foils were found to bend more despite their higher flexural stiffness [26], and this may be attributed to the larger median orbital defect area of 450.7 mm2 used in this study. Although the use of PDS 0.25 mm and PDS 0.5 mm resulted in similar degrees of overcorrection of the orbital height and foil bending, PDS 0.25 mm appeared to bend more and tended to restore orbital height more accurately than PDS 0.5 mm. In terms of the orbital volume, overcorrection that resulted in a significant difference between the reconstructed and native orbital volumes was observed when PDS 0.5 mm was used. However, the use of PDS 0.25 mm resulted in a restored orbital volume that was not significantly different from the initial volume. In summary, based on the restored orbital volume consistency and the quantitatively higher reconstructive accuracy of the orbital height and orbital volume, PDS 0.25 mm appeared to provide superior reconstructive results compared to PDS 0.5 mm in this study. As an inherent limitation of a study on cadaveric specimens, this study is blind to the occurrence of clinical symptoms following orbital reconstruction. Therefore, this study cannot make any statements about post-reconstructive complications, such as persistent double vision or enophthalmos, the occurrence of which would be decisive for the indication to perform secondary reconstructive measures [39]. Instead, this study deliberately investigated the geometric accuracy of orbital reconstruction using resorbable implants with different thicknesses. For this purpose, vertical height and orbital volume were used as surrogate parameters of orbital geometry, as vertical height and orbital volume define facial harmony [4, 5, 16]. Contrary to the surgical intuition to use thicker foil material to safely bridge the bony defect area, this study on cadaver specimens may allow the clinical translation to treat isolated orbital floor fractures with thinner foil material to ensure accurate restoration of facial harmony when there is an indication for the use of resorbable orbital implants.

The findings of this study also indicated that the use of PDS tended to result in an overcorrection of the orbital geometry. This could have been caused by a mismatch between the flat design of the foil and the convex–concave shape of the orbital floor. A discussion of different orbital implants for reconstruction of vertical height and orbital volume must consider the funnel geometry of the orbit and the anatomically complex, convex-concave shape of the orbital floor [44]. With resorbable foil material, the orbital height and the orbital volume seem to adapt naturally as the repositioned orbital contents naturally act on the flat and flexible foil material [23] and bend it depending on the foil thickness, as shown by the results of this study. Perforated or thermoplastic resorbable alloplastic material might be more shapable and therefore correspond more closely to the native orbital geometry [4, 45, 46].

Conventional titanium meshes must be bent by hand and might therefore lead to inaccuracies in the reconstruction of the vertical height and orbital volume [47]. Reconstruction with patient-specific orbital implants is based on the idealistic assumption that the vertical height and the orbital volume are identical on both sides, as the patient-specific implant corresponds to the mirror-image geometry of the orbital floor of the contralateral, healthy side [28, 48]. Ultimately, a disadvantage of rigid orbital implants could result from the fact that rigid orbital implants do not appear to allow natural adaption of the vertical height and orbital volume after reconstruction, but might instead permanently dictate the parameters of the orbital geometry due to the rigidity of the reconstruction material directly from the time of their insertion into the situs.

In terms of procurement, resorbable and standard preformed orbital implants are less expensive than individualized, patient-specific implants [28, 49]. In particular, due to the often comparable outcomes between expensive and cheap orbital implants, cost efficiency must be considered in order to conserve healthcare resources in the future [50]. Future research on resorbable reconstructive materials should focus on the development of thinner PDS foil material for the repair of small isolated orbital floor fractures [21] and posterior-impacted γ-shaped implants to prevent dislocation of resorbable reconstructive material in extended fractures [51].