Sagittal plane placement of the tibial component has important implications for maximizing the range of flexion motion [1] - [3] as well as providing ideal compressive loading on the tibial bone-prosthesis interface [4] [5] . The normal posterior tibial slope angle varies from Western to Eastern populations [1] [6] [7] . Similarly, the average post-operative TKR range of motion is considered good at 110˚ in the West, while this is inadequate in the Asian world, which requires 135˚ or greater flexion to allow normal squatting activities of daily living.

Anatomically, posterior slope of the tibial cut in the lateral plane improves available flexion for TKR and should be considered a routine part of the surgical procedure [1] [2] . This study quantifies the sagittal plane posterior slope angle in a Western osteoarthritic patient cohort both pre and post-operatively to re-introduce this important concept for current and future surgical applications.

Forty-nine primary cementless Buechel-Pappas (B-P) semi-constrained rotating-platform total knee replacements [5] (Endotec, Orlando FL) in 38 osteoarthritic patients (14 males and 24 females) from southwest Florida (Naples), were measured radiographically pre-operatively (pre-op) and 1 year post-operatively (post-op). Patient demographics included: age ranging from 40 to 91 years (mean 66 years), height ranging from 59 inches (150 cm) to 74 inches (188 cm) (mean 66 inches (168 cm)) and weight ranging from 130 lbs (58.5 kg) to 300 lb (135 kg) (mean 195 lbs (88 kg)). Each patient’s knee was examined radiographically in the lateral plane to determine the pre-op PTS angle using the anterior cortical line of the tibia as described by Yoo et al. [8] An extramedullary, adjustable tibial resection guide with a 7.5˚ posteriorly inclined cutting surface held with pins and an ankle clamp (B-P TKR Instrument System) (Endotec, Orlando, Florida) was used to make a perpendicular cut in the A-P plane and an anatomical, posteriorly inclined resection in the sagittal plane, see Figure 1 . Pre-op range of motion (ROM) and 1 year post-op ROM were evaluated with a goniometer and available radiographs to determine maintenance or improvement in knee motion.

The pre-op PTS was measured to be 11.83˚ (range 5˚ - 18˚) and the post-op PTS was 11.30˚ (range 4˚ - 18˚). Pre-op ROM in this study was measured to be 112˚ (range 30˚ to 135˚), which improved to 119˚ (range 90˚ to 135˚) post-op after 1 year. No loosening or component migration was seen in this study. Typical pre and post-op radiographs are shown in Figure 2 .

Active flexion following TKR is a critical outcome measure, especially for Asian patients that require 135˚ or greater flexion for their normal squatting and cross-legged sitting activities. Aside from using a prosthesis that has an inherent range of motion of 160˚ or more, it is important to consider the surgical technical aspects of TKR that can influence improved range of motion.

Surgeons have a direct ability to remove overhanging posterior femoral condylar bone and osteophytes that can impinge on prosthetic bearings and block flexion beyond 110˚. Surgeons can also influence flexion by using an anatomical posterior slope resection of the proximal tibia, which allows collateral ligaments to function more normally, than if a perpendicular resection is used [3] [7] . This is important since the tibial condylar surface is not perpendicular to the tibial axis. It is inclined posteriorly by approximately 11.4˚ as noted in a European population study by Brazier et al [9] and approximately 12.3˚ in a Nigerian population study by Didia et al. [10] Additionally, an anterior shear force on the bone-prosthesis junction, seen in perpendicular resections, is replaced by a compressive force with anatomical posterior slope [7] .

For these reasons, it seems important to quantify the normal posterior slope angle of the proximal tibia so it can be reproduced during tibial component implantation. Measurements of posterior tibial inclination (slope) vary depending upon the reference system used [6] [8] [11] . Computer navigation equipment may be able to give the intramedullary tibial-ankle loading axis in the lateral plane, but this is difficult for conventional extramedullary surgical techniques.

The recent introduction of Robotic Arm Assisted Computer navigated partial knee replacement [12] has provided the surgeon with real-time intra-operative assessment of PTS. Using this tool, surgeons can preoperatively assess the true PTS angle and then reproduce it with a robotically controlled cutting burr. Using this technique, surgeons have also found that reproduction of the patients anatomic posterior slope allows for the best ligament balancing throughout the entire range of motion [13] .

The intramedullary technique can give the approximate tibial-ankle axis [14] [15] , but canal invasion can produce unwanted fat emboli to challenge cardio-pulmonary function [16] - [18] .

Using anterior tibial shaft referencing, however, is easy and convenient, even in obese patients. A tibial resection guide with a 7˚ - 10˚ posteriorly inclined resection surface can be easily adjusted for greater or less inclination by moving the anterior alignment rod anteriorly or posteriorly, then locking the guide in position once the anatomical posterior slope has been achieved, see Figure 1 . This anterior tibial shaft referencing correlates with the lateral plane radiographs of this study, see Figure 2 . By aligning the tibial resection guide in line with the anatomical posterior slope, surgical resection has been simplified and quite accurate. The post-operative PTS of 11.30˚ seen in this study correlates well with the anatomical studies of Chiu et al [6] and Ishinishi et al [14] who determined the anatomic PTS to be 11.5˚ and 11.4˚, respectively.

The pre-operative range of motion of 112˚ (range 30˚ to 135˚) seen in this study was improved post-operatively to 119˚ (range 90˚ to 135˚) after 1 year. Similar improvement in range of motion using this technique has been reported in medium [19] and long term studies [20] - [23] , even though no difference in final range of motion was seen using a 0˚ or 5˚ posterior cutting block in the study of Kensara and Markel [24] .

Providing anatomical PTS of the proximal tibial cut in the sagittal plane is important for maintaining or improving range of post-operative motion and allowing collateral ligaments to function more normally as well as providing compressive loading rather than shear at the bone-prosthesis interface. The degree of posterior slope in this study pre-operatively measured 11.83˚ (range 5˚ - 18˚) and was surgically reproduced in the post-operative radiographs, which demonstrated a posterior inclination angle of 11.30˚ (range 4˚ - 18˚) using a conventional extramedullary tibial resection guide, routine lateral radiographs and a goniometer. Reasonable surgical approximation of PTS is achievable using anterior tibial shaft referencing and an adjustable extramedullary tibial resection guide.