## Facts

- A patentee owned a Japanese patent for an “infinite sliding ball spline bearing,” describing a ball-bearing structure that transmits torque while permitting continuous axial sliding.
- The asserted claim required, among other elements, (a) an outer cylinder with specified groove structures and end grooves and (b) a unitary holder structure enabling ball circulation.
- A manufacturer produced and sold “SPG-type” ball spline products during the 1980s.
- The accused products differed from the claim in at least two respects: (a) the groove configuration of the outer cylinder and (b) a divided holder system in which multiple components collectively performed functions attributed to the claimed holder.
- The trial court rejected infringement.
- The appellate court reversed, reasoning that despite literal differences, the accused structure achieved the same function through interchangeable means lacking special technical significance, and awarded relief.
- Some aspects of the accused configuration were disclosed in technology known publicly before the patent application.

## Issues

1. Whether a product that does not literally satisfy all claim elements can still fall within a patent’s technical scope under a doctrine of equivalents, and what conditions govern that determination.
2. Whether the accused ball spline products qualify as equivalents given the identified structural differences, potential prior-art disclosures at the filing date, and possible prosecution-related constraints.

## Decision

- The Supreme Court recognized a doctrine of equivalents under which a non-literal product may fall within the patent’s technical scope if five conditions are satisfied.
- The Supreme Court held the appellate court’s infringement analysis was legally insufficient, particularly as to the prior-art limitation and the absence of disqualifying “special circumstances.”
- The judgment finding infringement was reversed, and the case was remanded for further fact-finding and application of the five-part test.

## Legal Principles

- Patent scope is determined primarily by the claim language, but non-literal variants may be treated as within scope under an equivalents analysis subject to strict limits.
- Equivalence requires all of the following:
  - The differing part is not an essential element of the patented invention.
  - Replacing the claimed part with the accused part achieves the invention’s purpose and yields the same effect.
  - A person skilled in the art could readily conceive the substitution at the time the accused product was made.
  - The accused product is not identical to, and would not have been readily derived from, publicly known technology at the time of filing (preventing capture of unpatentable subject matter).
  - No disqualifying special circumstances exist, such as intentional exclusion of the accused configuration from the claims during prosecution (a form of prosecution-history-based estoppel).

## Conclusion

The Supreme Court established a five-condition doctrine of equivalents framework for patent infringement, then remanded because the lower court did not adequately analyze whether the accused design was barred by prior-art limits or by prosecution-related “special circumstances,” and did not sufficiently ground equivalence in the required factual findings.


THK K.K. v. Tsubakimoto Seiko Co., Ltd., 52 Minshū 113 (Sup. Ct. Japan 1998)