Total Knee Arthroplasty with CAOS Augmentation

Clinical Contributor

Ian Gradisar, MD
Crystal Clinic Orthopaedic Center
Akron, OH

Technical Contributors

Guillaume Bras, Dipl. Ing.
Blue Ortho

Amaury Jung, Dipl. Ing.
Blue Ortho

Yifei Dai, PhD


Computer-assisted Orthopedic Surgery (CAOS) offers increased accuracy to total knee arthroplasty (TKA) bony resections compared to the conventional techniques.1 Despite the proven benefits, one of the drawbacks for the adoption of CAOS technology may be the inconvenience of switching from conventional instruments to CAOS-specific instruments. A recent technology added CAOS augmentation to conventional mechanical instruments, removing the need for significant instrument relearning. The system has been shown to have a minimal learning curve2 and offers good usability and has been demonstrated to be non-disruptive to the surgical flow during its early adoption, reported by a subjective survey of users.3

As the clinical applications of this CAOS technology continue, further assessment is desired to confirm the promising results in a clinical setting. The purpose of this study was to collect and assess quantitative results regarding the use of the CAOS augmented technology* based on the combined experiences of a pilot surgeon group.

Materials and Methods

A retrospective review was carried out on the technical records of 411 primary TKA cases (no patient information was reviewed). All cases were performed by a group of three surgeons using conventional mechanical instrumentation augmented by the CAOS technology. Three analyses were performed on the technical records regarding:

Efficiency of the CAOS augmented technology: The time spent on landmark acquisition and CAOS guidance, as well as the total CAOS-guided surgical time were assessed.

Usage and scope of intra-operative CAOS guidance: The location of the initially placed tibial and femoral cutting blocks were reviewed, from which the percentage of cases with the cutting block initially placed at greater than 2° varus/valgus was calculated. When there was a need to correct the initial placement of the cutting block, the surgeon turned an adjustment knob on the block with each turning “click” corresponding to a one-degree or -mm adjustment increment. The amount of adjustment in the coronal/sagittal alignments and resection depth were assessed.

Accuracy of the bony resection: The intra-operatively measured deviations between the surgeon’s set resection goal and the achieved bony resection (coronal and sagittal alignments and resection depth) were calculated to investigate resection accuracy.


On average, it took one minute or less for the acquisition of the anatomic landmarks on the tibia and femur, respectively (Table 1A). Similar results were found in the time spent on CAOS guided tibial and femoral cutting block adjustments. The total CAOS guided surgical time was found to be 11.1 ± 4.4 minutes per case, with 97.0% and 99.1% of the cases required no more than 20 minutes and 30 minutes, respectively.

In 19.8% and 22.4% of the cases, the initial cutting block placements had more than 2° error in varus/valgus alignment, respectively for the tibia and femur. The percentage reduced drastically to ~1% after the surgeon adjusted the cutting block based on the CAOS guidance. The scope of adjustment for all 411 TKA cases was summarized in Table 1B. Generally, each knee required correction on both alignment and depth parameters. Results showed a similar amount of corrections across coronal alignment (3.3° ± 3.0°), sagittal alignment (3.6° ± 3.0°), and resection depth (3.5mm ± 3.7mm) (n.s.) of the knee. When comparing bone types, the adjustment of orientation and position of the femoral cutting block required more corrections compared to the tibial cutting block. Further analysis revealed that more corrections were applied to adjust the sagittal alignment of the femoral cutting block compared to those required for the tibial cutting block. High resection accuracy was achieved for both tibial and femoral resections (Table 1C).


This study reported the prevalence of inaccuracy in the position of manually placed cutting blocks during conventional TKA surgeries. It was observed that 20% of the time, the surgeons placed the cutting block with more than 2° deviation from the ideal coronal alignment, potentially impacting clinical results.4 Adding CAOS augmentation to mechanical instrumentation was demonstrated to substantially improve resection accuracy. With CAOS guidance, considerable adjustments were easily achieved to correct the cutting blocks to the proper position. Significantly more adjustment was needed in femoral flexion/extension compared to tibial posterior slope. This might be caused by the inaccuracy of the intramedullary reamer placement used to establish flexion/extension alignment to the mechanical axis, especially with the prevalence of femoral sagittal bowing. The results also demonstrated minimal impact on the surgical efficiency (time) from incorporating the CAOS augmented technology into the surgical workflow.

While previous studies reported only 70% of conventional TKAs could achieve acceptable alignment accuracy,5,6 the findings from this study confirmed and emphasized the clinical need for mitigating error during manual conventional bony preparation, which can be addressed efficiently and accurately with CAOS-augmented mechanical instrumentation.

Table 1.
A) Efficiency, B) Resection Accuracy and C) Scope of CAOS-guided adjustments in the TKA cases performed with CAOS-augmented mechanical instrumentation.

Figure 1. Percentage distribution of cases regarding the number of “clicks” (°/mm adjustment) required to correct the position of initially placed A) tibial and B) femoral cutting blocks.

Computer-assisted Orthopedic Surgery (CAOS) offers increased accuracy to total knee arthroplasty (TKA) bony resections compared to the conventional techniques.1

*The technology (ExactechGPS® TKA Plus) is a CT-free application designed to provide CAOS augmentation to conventional mechanical instrumentation. The system involves direct placement of the localizers onto the conventional femoral and tibial cutting guides. This system offers surgeons streamlined guidance for the proximal tibial and distal femoral cuts with minor change of instrumentation and surgical workflow.


  1. Sparmann M, et al. Po­sitioning of total knee arthroplasty with and without naviga­tion support: a prospective, randomised study. J Bone Joint Surg Br. 2003;85(6):830-5.
  2. Dai Y, et al. Learning of a caos enhanced mechanical instrument system for total knee arthroplasty: a cusum analysis. CAOS 2018
  3. Dai Y, et al. Early experience with CAOS enhanced total knee arthroplasty – a global, multi-surgeon evaluation. CAOS 2019
  4. Walker PS, Meere PA, Bell CP. Effects of surgical variables in balancing of total knee replacements using an instrumented tibial trial. 2014;21(1):156-61
  5. Ritter MA, et al. Preoperative malalignment increases risks of failure after total knee arthroplasty. J Bone Joint Surg Am. 2013;95(2):126-31.
  6. Hetaimish BM, et al. Meta-analysis of navigation vs conventional total knee arthroplasty. J Arthroplasty. 2012;27(6):1177-82.