Subsidence in LLIF

Fig. 28.1

Early cage subsidence (ECS) described by Malham et al. [9]

Fig. 28.2

Three types of subsidence (Adapted from Malham et al. [9])

28.3 Rates

The rate of subsidence after LLIF has been reported as 10–62 % [2, 4–8, 10]; however, the inconsistencies in evaluating, defining and reporting subsidence after this procedure make the significance of such findings unclear. In our early experience with LLIF, we reported a radiographic subsidence rate of 13 % (4 of 30 patients) using CT [11]. In the next 40 cases, the subsidence rate was 7.5 % (3 of 40), with all cases occurring in patients with standalone cages [12]. In a later series of 128 patients (178 levels), we reported a subsidence (DCS) rate of 10 % (13 of 128) of patients in 8 % (14 of 178) of levels [9]. Tohmeh et al. [2] reported higher rates of ECS and DCS with 20.2 % (45 of 223) of cages immediately postoperatively and 62.3 % (139 of 223) at 12 months.

Clinical (symptomatic) subsidence occurred in 3 % (4 of 128) of our patients. The depth of subsidence in this series ranged from 1.6 to 6.0 mm. Type 2 subsidence was the most common in 64 % (9 of 14 levels), followed by type 3 in 21 % (3 of 14 levels) and type 1 in 14 % (2 of 14 levels). Four cases of ECS were identified, all of which corresponded to type 2 subsidence. Le et al. [1] reported a similar rate of subsidence with 14.3 % (20 of 140) of patients in 8.8 % (21 of 238) of levels and clinical subsidence in 2.1 %. The depth of subsidence ranged from 2 to 9 mm.

28.4 Risk Factors

28.4.1 Caudal Endplate

The caudal endplate is 40 % weaker than the cranial endplate [13] and thus is at higher risk of subsidence. This has been confirmed in most case series reported [1, 2, 8, 9]. In addition, the central regions are thinner and weaker than the peripheral regions of lumbar endplates [14].

28.4.2 Level

Although lumbar endplate strength increases from L1–2 to L4–5 [15], L4–5 is technically the most challenging level in LLIF because the lumbar plexus can force an anterior cage position. Also the height of the iliac crest may prevent parallel trial and cage insertion, and even with angled instruments, the force vector is not parallel, but into the weaker caudal endplate. Marchi et al. [7] reported the L4–5 level to have the highest rate of subsidence. We had a similar experience with 71 % of subsidence occurring at the L4–5 level [9].

Regarding specific levels, Le et al. [1] found subsidence rates of 20 %, 10.4 % and 4.1 % at L1–L2, L2–L3 and L3–L4, respectively. Although this was not statistically significant, it is consistent with endplate strength increasing in the lower levels of the lumbar spine and the subsidence rate at L4–5 of 10.3 % may relate to the technical factors described above.

28.4.3 Bone Quality

A lower bone mineral density (BMD) leads to a lower failure load of the vertebrae; this increases the subsidence risk, especially with severe osteoporosis [14, 16]. Patients with decreased BMD still have the same failure load distribution as patients with normal BMD [14]. Thus, in these instances it is important to place the cage where there is the greatest resistance to subsidence. The relationship between BMD scores and graft subsidence was examined by Tempel et al. [5]. The mean DEXA T-score in patients with subsidence was −1.65 (SD 1.04) compared to −0.45 (SD 0.97) in patients without subsidence (P < 0.01).

28.4.4 Cage Size

Cage width is an important factor in resisting subsidence. LLIF enables insertion of a wide footprint cage spanning the endplate apophyseal rim. The larger surface area of the 22- and 26-mm cages cover more of the stronger peripheral endplate [17] and lead to more efficient transfer of force to the endplate than that of the narrower 18 mm cage. Significantly higher subsidence rates using 18 mm cages compared to 22 mm cages were reported by Le et al. [1], with 14.1 % and 1.9 %, respectively (P < 0.0001). Subsequent studies have confirmed similar results [2, 8, 9].

The greater the cage height, the higher the rate of subsidence [2, 18]. Both Le et al. [1] and Malham et al. [9] restricted cage height to 8–12 mm in a conscious effort to avoid overdistraction and subsequent endplate violation. Importantly, limiting disc height to this amount still provided adequate indirect decompression [19].

The cage length may only be relevant if is not sufficient to cover the periphery of the endplates [13]; this was confirmed by Le et al. [1] who found that implant length had no effect on subsidence.

28.4.5 Bone Morphogenetic Protein

Recombinant bone morphogenetic protein-2 (rhBMP-2) can be used as a bone graft substitute in LLIF that avoids iliac crest harvest and provides high fusion rates without cancer risk [20]. Theoretically, rhBMP-2-related osteolysis is of concern in the first four to six postoperative weeks because of bone softening in the initial rhBMP-2-induced osteoclastic inflammatory response and resorption phase prior to osteoblastic bone formation and consolidation [21, 22].

28.5 Effect on Clinical Outcomes and Fusion Rates

Subsidence has been shown to have no influence on bone fusion rates [9]; however, it may contribute to early postoperative pain [7, 8, 23]. Some surgical goals may not be achieved in cases of subsidence, namely, mechanical stabilisation, correction of sagittal/coronal alignment, distraction of the disc space and decompression of neural elements [1, 23]. However, studies comparing subsidence to final clinical outcomes have not identified a clear relationship [7, 8]. We found that neither interbody fusion rates nor clinical outcomes were affected by radiographic subsidence [9]. Despite a significant difference in fusion rates between the subsidence and non-subsidence groups at 6 months (0 % and 30 %, respectively; P = 0.0195), by 18 months, the fusion rates for both groups were similar (73 % and 88 %, P = 0.1792). Sharma et al. concurred that subsidence did not affect fusion rates [3].

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