Radiographic and clinical evidence: osteoarthritic knee can change surgical result for lumbar degenerative disease patient undergone surgery for 3-year follow-up: a retrospective comparative clinical study

Degeneration of spine and arthritis of knee may be a natural course in ambulating upright human body and the number of patients with problems in both spine and knee is increasing with rising life expectancy in many countries [1]. Many aged patients who undergo spinal surgery frequently present symptoms relating to knee osteoarthritis (OA). Therefore, failure to recognize a concurrent disease may lead to misdiagnosis and possibly erroneous treatment. Although it is difficult to decide what might be the main pathology in order of priority, we sometimes observe the improved spinal alignment or symptoms after elimination of knee problem by total knee arthroplasty [2].

Although there are still no definite studies investigating knee joints’ effect on spinopelvic alignment, Tsuji et al. [3] introduced the knee-spine syndrome: phenomenon of thigh muscle tightness and knee flexion leading decreased lumbar lordosis and sacral inclination while standing in elderly Japanese. And also, Takemitsu et al. [4] previously reported that patients of lumbar degenerative kyphosis with dorsal tilted sacrum were standing in knee-flexion position to gain mechanical advantages. Because motion of the knee joint has such a significant impact on the biomechanics of sagittal balance, it is also required to understand the effect of knee OA on the outcomes of patients who undergo spinal fusion surgery.

To our knowledge, there is a paucity of reports clarifying the implication of knee osteoarthritis on spinal sagittal alignment of patients undergoing posterior instrumentation and fusion. The purpose of this study is thus to evaluate how osteoarthritic knee affects spinal sagittal alignment of degenerative lumbar disease patients undergoing posterior instrumentation and fusion using time dependent outcome analysis.

Ambulatory humans should maintain upright standing position with well-aligned weight bearing segments to achieve the minimization of energy expenditure [9, 10]. Not to mention, the sagittal spinal alignment is crucial in maintaining well-aligned standing position. However, if any pathological or degenerative changes in the spine, pelvis or lower extremities occurs, they would disrupt the interactive balanced posture; then, the spine–pelvis–leg alignment should be restored by compensatory changes in other segments. Sagittal spinopelvic alignment and compensatory mechanisms in patients with spinal disorders were reported regarding a relationship between pelvis and spine, and it has been well known that spinal sagittal imbalance leads to adaptive changes in the pelvis, hip joint through compensatory mechanism [2, 11‐14]. And also, knee flexion [15] is also well-known compensatory mechanism accompanied by ankle extension (dorsiflexion) in sagittal plane. Aside from compensation of spine itself, therefore, postoperative symptoms arising from the lumbar spine might be insufficient compensatory mechanism caused indirectly by hip [16, 17] and knee osteoarthritis (OA) [6, 18], et cetera. Knee OA can be easily assessed in coronal plane radiographs such as standing anteroposterior or 45-degree flexed posteroanterior (Rosenberg’s) view and we used the former method.

In the current study, TLK, LL and PT in both groups improved at postoperative 1 month. Whereas those results achieved at 1 month in group I were maintained until the ultimate follow-up, LL and PT in group II showed deterioration at the ultimate follow-up. Particularly, the interval changes in LL from the preoperative to the ultimate follow-up and from postoperative 1 month to the ultimate follow-up were significantly different between the groups showing inferior outcome in group II. This is considered postoperative LL was more closely and directly associated with severe knee OA than other spinopelvic parameters. It is keeping with the previous literatures that the knee OA with flexion contracture can lead to decreased lumbar lordosis [2, 18] and PT will not be directly correlated to LL if lumbar spine is flexible [2]. In normal compensation mechanism, postoperatively restored LL should have result in decreased PT and decreased knee flexion. However, in severe knee OA such as group II, we think this mechanism does not seem to work normally. The phenomenon can be explained by the literatures investigating biomechanical incapability of OA knee [9, 19]. Messier et al. [19] suggested that patients with knee OA reduce the knee extension moments and Astephen et al. [9] also reported decreased early stance knee extension moments of progressive OA in biomechanical analysis related with knee OA severity. On the correlation of knee and LL, Murata et al. [18] reported significantly reduced LL in patients whose limitation of knee extension was more than 5 degrees. And Lee et al. [2] also reported decreased PT and increased sacral slope after total knee arthroplasty in patients with preoperative knee flexion contracture more than 10 degrees. Presumably, although restored LL after spinal surgery allow the margin of compensation to tilted pelvis and flexed knees, flexion contracture of OA knee will prohibit knee extension, subsequent limitation of motion in hip and ankle joint.

However, CL and TK were not different between the groups postoperatively. We think cervical and thoracic spine is located so distant from the knee joints compared to lumbar spine that they cannot be easily affected by knee OA. Another reason to explain this is the compensatory changes in TK cannot work well in older patients because thoracic hypokyphosis requires strong muscle tone, which maybe deficient in older patients [20]. Also, it is because cervical lordosis in older patients is usually considered stiff and already recruited to maintain horizontal gaze [20].

Regarding global sagittal balance, C7SVA, CrSVA-H and CrSVA-H improved at postoperative 1 month showed aggravation at the ultimate follow-up in group II. Despite the aggravation in group II until the ultimate follow-up, C7SVA was still significantly different between the preoperative and the ultimate follow-up. But we found no significant differences between the preoperative value and the ultimate follow-up value in CrSVA-S and CrSVA-H, which means C7SVA is less sensitive than CrSVA-S and CrSVA-H in detecting mild deterioration of global sagittal balance. It was also reported CrSVA was more correlated with ODI and all SRS subscores than C7SVA [7], which could not consider the motion of cervical spine [21]. But we still think further investigation of cranial parameters is warranted to clarify their meaning as global alignment parameters.

However, even in patients with insufficient sagittal correction after spinal fusion surgery, postoperative sagittal imbalance often improves with time [22]. Although it is not always clearly understood, it can be sometimes explained by the improvement of pain and increased function after laminectomy facilitating patients’ restoration of upright posture [12]. In our study, the preoperative score of SRS pain and function significantly improved in both groups at the ultimate follow-up. However, group I showed significantly better outcomes than group II in comparison of the amount of improved ODI score (21.5 vs 32.9, p < 0.001), SRS total (%) (78.8 vs 67.6, p = 0.037), improved SRS function (1.4 vs 0.7, p = 0.016), and satisfaction (4.2 vs 3.0, p < 0.001). Moreover, although group II showed improvement at the ultimate follow-up in ODI, SRS pain, self-image and function scores, their satisfaction and mental health score were not improved compared to the preoperative. These results indicate that clinical prognosis after spinal fusion surgery may be unfavorable if severe knee OA in patients exists concurrently. According to the report of Ho et al. [23], at postoperative 1 year, ODI scores were shown to be affected by the operational level, the preoperative ODI, and the presence of advanced radiographic knee OA (Kellgren/Lawrence grades III and IV) (P < 0.05). This phenomenon was also supported in the study of the relationship between clinical outcomes and sagittal radiographic parameters by Lafage et al. [24]. Therefore, when spine surgeons plan spinal instrumented fusion surgery in patients with osteoarthritic knees, the severity of osteoarthritic knee should be evaluated preoperatively and simultaneous treatment of knee OA should be considered in patients who are required to undergo spinal instrumented fusion surgery.

In summary, regardless of the severity in preoperative knee OA, the initial improvement in radiographic measurement and clinical outcomes can be achieved by spinal surgery not only with instrumented correction of spinal alignment but also with the spinal decompression. However, patients with knee OA of K-L grade 3 or 4 were lacking in compensatory mechanism by knee joint motion maintaining immediate postoperative spinal sagittal balance. The most important mechanism we explained was accompanying knee flexion contracture in OA knee which had less capacity of extensive joint motion to accommodate improved spinal balance.

We acknowledge that this study has several limitations. To elucidate the effect of the knee OA on maintaining the immediate postoperative outcomes of spinal surgery, whether the patients with knee OA of KLG 3 or 4 underwent knee replacement surgery would have shown the importance of knee OA directly. Besides, regrettably, knee radiographs were not obtained in all of recruited patients during postoperative follow-up. Because it can be pointed out there are uncertainty that knee OA severity was aggravated from K-L grade 1 or 2 to K-L grade 3 or 4 in some patients of the group I during the postoperative follow-up. Moreover, the retrospective design introduced a degree of uncertainty due to some missing and erroneous data in medical records. In addition, as is already well known, there is little relationship between structural severity in knee joints’ pathology and clinical symptoms. Potentially, not only knee osteoarthritis but also other factors [11] including ongoing degeneration in spine itself or combined with knee issue or problem in knee only or related to muscle imbalance after surgery could affect post-surgical sagittal balance and clinical improvement. Therefore, grouping by K-L grading itself cannot represent the precise effect of osteoarthritic knee on compensatory capacity.

Osteoarthritic knee with Kellgren-Lawrence Grading Scale of 3 or 4 have a negative influence on maintaining postoperative spinal sagittal alignment, balance, and clinical outcomes achieved immediately by posterior instrumentation and fusion for lumbar degenerative disease.