In recent years, fracture and implant fixation in the elderly patients with reduced bone quality gained more and more attention. New concepts range from alternative fixations options such as transfacet screws or lamina hooks [10, 11] to expandable or modified pedicle screws [12,13] and to bone cements with modified, more physiological mechanical properties [14, 15]. For conventional pedicle screws, various augmentation techniques and augmentation materials to prevent screw loosening are available [16]. So far, the augmentation of pedicle screws with PMMA has demonstrated to be the most efficient solution to enhance screw strength in osteoporotic bone [2, 3, 4, 5, 6, 7]. It has shown clinical efficacy in the management of degenerative disease [17] and fractures [18, 19, 20]. However, there is a concern about the ideal time for augmentation during the surgical procedure, especially if a restoration or a derotation maneuver of the instrumented vertebra is part of the surgery.
The results of the present study have demonstrated that if a reposition maneuver or derotation is conducted, the time of cement application has an effect on pedicle screw anchorage in the osteoporotic vertebral body. Therefore, both hypotheses can be confirmed.
In comparison with pedicle screws augmented prior to the reposition maneuver (Aug_Man), pedicle screws augmented after the reposition maneuver (Man_Aug) showed a significant higher number of load cycles until failure. Pedicle screws augmented after the reposition maneuver (Man_Aug) had a smaller per cycle motion during loading than pedicle screws augmented prior to the reposition maneuver (Aug_Man). The predominant loosening mechanism for pedicle screws augmented after the reposition maneuver (Man_Aug) was pedicle screw toggling with the attached cement cloud within the trabecular bone. While pedicle screws augmented prior to the reposition, maneuver (Aug_Man) showed a motion of the screw within the cement cloud. However, the loosening mechanism for the Aug_Man group might also show predominantly a loosening with the cement cloud attached to the screw if the cement is fully cured during the reposition maneuver.
Some studies have compared the pull-out strength of the screws with the timing of PMMA application. Flahiff et al. compared the axial pull-out strength of pedicle screws inserted in “soft,” “doughy,” or “hard” PMMA cement in synthetic bone [21]. Screw placement in “doughy” PMMA resulted in the highest increase in pull-out forces, while placement in the hard cement had the smallest effect compared to the non-augmented control group. Linhardt et al. showed a more pronounced increase in axial pull-out strength for screws placed immediately after cement injection in doughy cement than for placement of the screws 60 min after cement injection [22]. For screw placement in doughy cement, the failure occurred at the bone–cement interface, while for screw placement after 60 min of cement injection, it occurred at the screw–cement interface. This might be due to better bonding of the cement with the screw threads for immediate screw placement in doughy cement. The failure mode of screw placement in doughy cement at the bone–cement interface is comparable to the Man_Aug group of the present study. The screw was also surrounded by doughy cement, fully bonded to the cement, and predominantly showed a failure at the cement–bone interface with the screw bonded to the cement toggling within trabecular bone. In contrast to this in the Aug_Man group, the cement–screw bonding was likely disrupted by the reposition maneuver and caused a loosening at the cement–screw interface with motion of the screw within the cement cloud still rigidly fixed to the trabecular bone.
However, these studies used a pull-out test instead of cyclic cranio-caudal loading and did not investigate the effect of cement application timing in combination with a reposition maneuver a reposition maneuver on screw anchorage.
In order not to further compromise pedicle screw anchorage in patients with reduced bone quality it is recommended to avoid correction maneuvers based on the instrumentation in patients with reduced bone quality [23]. During a reposition maneuver in combination with screw augmentation the screw anchorage can be compromised and cause clinical failure of the instrumentation by disengagement of the screw from the PMMA cement with the cement still attached to the bone [24]. To avoid compromising the screw anchorage, screws can be augmented after the reposition maneuver or in cases with augmentation prior the reposition maneuver sufficient time should be taken to allow the cement to fully cure.
The carried out reposition maneuver was standardized and intended to simulate a fracture reposition with a pivot point close to the pedicle. Due to varying lever arm ratios in reposition tools and techniques, forces measured during the simulated reposition maneuver cannot be directly transferred to clinical practice. It might be estimated that to obtain comparable reposition forces on the pedicle screw with various reposition instruments used in clinical practice only 20–50% of the forces reported with the present setup have to be applied by the surgeon to the reposition instrument.
A limitation of the study is that time points for the reposition maneuver after the augmentation were not varied and therefore an adequate waiting time for the reposition maneuver after augmentation can not be stated. The timespan was chosen similar to the clinical setting to achieve a good transferability into clinical practice. However, the required waiting time will likely vary depending on the brand and manufacturer of the PMMA cement.
Another limitation in this study is that it was conducted on cadaver specimens, with their inherent limitations. Cadaver specimens cannot replicate biological factors such as bone remodeling around PMMA cement. Osseous resistance, loading and in vivo loading of structures can only be approximated.
This study’s strengths are that its findings underline for the first time, that the time of cement application has an effect on pedicle screw anchorage in the osteoporotic vertebral body under physiologic related cyclic cranio-caudal loading [2, 25, 26, 27, 28, 29] and it applies a stepwise increasing load protocol designed to investigate implant anchorage in reduced bone quality [30, 31].